Method for producing acrolein and/or acrylic acid

ABSTRACT

Acrolein and/or acrylic acid are prepared from propane and/or propene by a process comprising the following steps: (a) separation of propane and/or propene from a propane- and/or propene-containing gas mixture by absorption in an absorbent, (b) separation of the propane and/or propene from the absorbent to give a gas B and (c) use of the gas B obtained in stage (b) for an oxidation of propane and/or propene to acrolein and/or acrylic acid, no heterogeneously catalyzed dehydrogenation of propane without supply of oxygen being carried out between steps (b) and (c).

[0001] The present invention relates to a process for the preparation ofacrolein and/or acrylic acid from propane and/or propene.

[0002] Acrolein and acrylic acid are key chemicals. Thus, acrylic acidis used, inter alia, as a monomer for the preparation of polymers which,for example as a dispersion in an aqueous medium, are used as binders.Depending on the intended use of the polymer, an esterification of theacrylic acid may also take place before the polymerization. Acrolein isan important intermediate, for example for the preparation ofglutaraldehyde, methionine, folic acid and acrylic acid.

[0003] Known processes for the preparation of acrolein and/or acrylicacid start from propane and/or propene. DE-A 33 13 573 and EP-A-0 117146 disclose a process for converting propane into acrolein and/oracrylic acid. A two-stage or three-stage process in which the propane isdehydrogenated to propene in a first stage and the propene is oxidizedto acrolein in a second stage is described. An important feature here isthat no separation of the propane from secondary components formed inthe dehydrogenation, for example molecular hydrogen, is carried outbetween the two stages. The oxidation is carried out under conditionsunder which no marked oxidation of the hydrogen takes place. In a thirdstage, the acrolein can then be oxidized to acrylic acid. It is alsopossible to separate off unconverted propane and propene from the secondor third stage by absorption and, after liberation from the absorbent,to recycle them to the first stage (dehydrogenation stage).

[0004] Japanese Patent Application JP-A 10-36311 discloses a process forthe preparation of α,β-unsaturated carboxylic acids, such as acrylicacid, by gas-phase catalytic oxidation of propane in the presence of acomposite metal oxide catalyst, the ratio of propane to oxygen and, ifrequired, diluent gas being kept in a defined range for achieving highyields in the starting mixture and furthermore the conversion being keptat a specific value. Unconverted propane can be separated off after theoxidation by a selective separator which comprises pressure swingadsorption units and then recycled to the gas-phase catalytic oxidation.

[0005] GB 1378-178 discloses a process in which unconverted hydrocarbonfrom an oxidation process is absorbed in an absorbent and the absorbentis stripped with a stripping medium. In this, the hydrocarbon to berecovered is added to the stripping medium in such a quantity that themixture is outside the flammable limits.

[0006] It is an object of the present invention to provide a process forthe gas-phase catalytic preparation of acrolein and/or acrylic acid frompropane and/or propene, which is economical and in which the catalystused can be employed for a very long time without regeneration.

[0007] We have found that this object is achieved, according to theinvention, by absorption of propane and/or propene from a propane-and/or propene-containing gas mixture into an absorbent, separation ofthe propane and/or propene from the absorbent and subsequent use of thepropane and/or propene for an oxidation to acrolein and/or acrylic acid.

[0008] The present invention therefore relates to a process for thepreparation of acrolein and/or acrylic acid from propane and/or propene,the process comprising the following steps:

[0009] (a) separation of propane and/or propene from propane- and/orpropene-containing gas mixture A by absorption into an absorbent,

[0010] (b) separation of the propane and/or propene from the absorbentto give a propane- and/or propene-containing gas B and

[0011] (c) use of the gas B obtained in step (b) for an oxidation ofpropane and/or propene to acrolein and/or acrylic acid,

[0012] no heterogeneously catalyzed dehydrogenation of propane without asupply of oxygen being carried out between step (b) and step (c).Preferred embodiments of the invention are evident from the followingdescription and the figures.

[0013] Since the propane and/or propane are subjected to absorptionbefore the oxidation, as a rule residues of absorbent are present in thegas B. Surprisingly, it has now been found that nevertheless no problemsoccur during the oxidation. Thus, no substantial decrease in theactivity of the oxidation catalyst was observed, and the oxidationcatalyst could be used over a long operating period withoutregeneration. Furthermore, no problems due to any expected concomitantoxidation of residues of absorbent in the oxidation stage were observed.Where residues of the absorbent present problems, which is generally notthe case when hydrocarbons having a high boiling point, in particularparaffins, are used as the absorbent, said absorbent can be removed, forexample by quenching with water or by adsorption.

[0014] In the process according to DE-A 33 13 573, propane and propenerecovered by absorption or separated off are recycled to theheterogeneously catalyzed dehydrogenation of propane. In theheterogeneously catalyzed propane dehydrogenation, the catalyst may bedeactivated, for example by coking. Such dehydrogenation catalysts aretherefore frequently regenerated. The absorbent fed in together with thegas stream therefore presents no problems since it can be incineratedtogether with the coke. On the other hand, the catalysts used in theoxidation to acrolein and/or acrylic acid are usually not regenerated sooften because the additional regeneration cost which arises by virtue ofthe fact that the feed gas contains absorbent is greater than in thecase of dehydrogenation. The novel process has the advantage that theoxidation catalyst can be used over a long period without regeneration.

[0015] The novel process differs from the process according to DE-A 3313 573 in that the propane and/or propene separated off by absorptionis/are fed to an oxidation stage. In contrast to the situation in theprocess according to DE-A 33 13 573, there is, according to theinvention, no heterogeneously catalyzed dehydrogenation of propanewithout supply of oxygen between the separation of propane and/orpropene from the absorbent and the oxidation to acrolein and/or acrylicacid.

[0016] In the present invention, the gas B may also be a gas mixture.

[0017] In step (a), gas mixtures A comprising any desired amounts ofpropane and/or propene can be used. Preferably, the gas mixture Acontains propane and propene in a molar ratio of from 0:100 to 100:0, inparticular from 10:90 to 90:10, frequently from 80:20 to 40:60.

[0018] Preferably, the gas mixture A contains at least one furthercomponent which differs from propane and/or propene and is not subjectto any particular restrictions. As a rule, the further components dependon the origin of the gas mixture. In particular, they comprise at leastone component selected from nitrogen, hydrogen, oxides of carbon, suchas carbon monoxide or carbon dioxide, further secondary componentsoriginating from a propane dehydrogenation, secondary componentsoriginating from a gas-phase oxidation of propene to acrolein and/oracrylic acid or secondary components originating from an oxidation ofpropane to acrolein and/or acrylic acid. Frequently, at least hydrogen,nitrogen, an oxide of carbon or a mixture of these is present as afurther component.

[0019] Suitable absorbents in step (a) are in principle all absorbentswhich are capable of absorbing propane and/or propene. The absorbent ispreferably an organic solvent which is preferably hydrophobic and/orhigh-boiling. Advantageously, this solvent has a boiling point (at anatmospheric pressure of 1 atm) of at least 120° C., preferably of atleast 180° C., especially from 200 to 350° C., in particular from 250 to300° C., more preferably from 260 to 290° C. Expediently, the flashpoint(at an atmospheric pressure of 1 atm) is above 110° C. In general,suitable absorbents are relatively nonpolar organic solvents, forexample aliphatic hydrocarbons which preferably contain no externallyacting polar group, but also aromatic hydrocarbons. In general, it isdesirable for the absorbent to have a very high boiling point incombination with very high solubility for propane and/or propene.Examples of absorbents are aliphatic hydrocarbons, for exampleC₈-C₂₀-alkanes or C₈-C₂₀-alkenes, or aromatic hydrocarbons, for examplemiddle oil fractions from paraffin distillation or ethers having bulkygroups on the oxygen atom, or mixtures thereof, it being possible to addto them a polar solvent, for example, the 1,2-dimethyl phthalatedisclosed in DE-A 43 08 087. Esters of benzoic acid and phthalic acidwith straight-chain alkanols of 1 to 8 carbon atoms, such as n-butylbenzoate, methyl benzoate, ethyl benzoate, dimethyl phthalate anddiethyl phthalate, and thermal oils, such as biphenyl, diphenyl etherand mixtures of biphenyl and diphenyl ether or their chlorinederivatives and triarylalkenes, for example4-methyl-4′-benzyldiphenylmethane and its isomers2-methyl-2′-benzyldiphenylmethane, 2-methyl-4′-benzyldiphenylmethane and4-methyl-2′-benzyldiphenylmethane, and mixtures of such isomers arefurthermore suitable. A suitable absorbent is a solvent mixturecomprising biphenyl and diphenyl ether, preferably in the azeotropiccomposition, in particular comprising about 25% by weight of biphenyland about 75% by weight of diphenyl ether, for example the commerciallyavailable diphyl. Frequently, this solvent mixture contains an addedsolvent, such as dimethyl phthalate, in an amount of from 0.1 to 25% byweight, based on the total solvent mixture. Other particularly suitableabsorbents are octanes, nonanes, decanes, undecanes, dodecanes,tridecanes, tetradecanes, pentadecanes, hexadecanes, heptadecanes andoctadecanes, tetradecane having proven particularly suitable. It isadvantageous if the absorbent used on the one hand has theabovementioned boiling point but on the other hand simultaneously hasnot too high a molecular weight. Advantageously, the molecular weight ofthe absorbent is ≦300 g/mol. The liquid paraffins of 8 to 10 carbonatoms which are described in DE-A 33 13 573 are also suitable. Examplesof suitable commercial products are products sold by Haltermann, such asHalpasols i, e.g. Halpasol 250/340 i and Halpasol 250/275 i, andprinting ink oils with the names PKWF and Printosol.

[0020] The absorption procedure is not subject to any particularrestrictions. All processes and conditions familiar to a person skilledin the art may be used. Preferably, the gas mixture is brought intocontact with the absorbent at from 1 to 50, preferably from 2 to 20,more preferably from 5 to 10, bar and from 0 to 100° C., in particularfrom 30 to 50° C. The absorption can be carried out both in columns andin quench apparatuses. The cocurrent or the countercurrent procedure maybe employed. Suitable absorption columns are, for example, tray columns(having bubble trays and/or sieve trays), columns having stackedpackings (for example sheet metal packings having a specific surfacearea of from 100 to 500 m²/m³, for example Mellapak® 250 Y) and columnshaving dumped packings (for example filled with Raschig packings).However, trickle and spray towers, graphite block absorbers, surfaceabsorbers, such as thick-film and thin-film absorbers, and platescrubbers, cross-spray scrubbers and rotary scrubbers may also be used.It may also be advantageous to allow the absorption to take place in abubble column with or without internals.

[0021] The separation of the propane and/or propene from the absorbentcan be effected by stripping, flashing and/or distillation.

[0022] The separation of the propane and/or propene from the absorbentin step (b) is preferably effected by stripping or desorption with a gaswhich is inert with respect to the novel step (c) and/or with molecularoxygen (for example air). Here, the stripping can be carried out in theusual manner via a pressure and/or temperature change, preferably atfrom 0.1 to 10, in particular from 1 to 5, more preferably from 1 to 2,bar and from 0 to 200° C., in particular from 20 to 100° C., morepreferably from 30 to 50° C. Another gas suitable for the stripping is,for example steam, but oxygen/nitrogen mixtures are particularlypreferred, for example air. With the use of air or oxygen/nitrogenmixtures in which the oxygen content is more than 10% by volume, it maybe expedient to add, before or during the stripping process, a gas whichreduces the explosion range. Particularly suitable for this purpose aregases having a specific heat capacity of >29 J/mol·K at 20° C., forexample methane, ethane, propane, butane, pentane, hexane, benzene,methanol, ethanol, ammonia, carbon dioxide or water. Bubble columns withor without internals are also particularly suitable for the stripping.

[0023] The separation of the propane and/or propene from the absorbentcan also be effected by means of distillation, it being possible to usethe columns familiar to a person skilled in the art and containingstacked packings, dumped packings or corresponding internals.

[0024] Preferred conditions during the distillation are a pressure offrom 0.01 to 5, in particular from 0.1 to 3, more preferably from 1 to2, bar and a temperature (at the bottom) of from 50 to 300° C., inparticular from 150 to 250° C.

[0025] Where the gas mixture A contains water, the absorption isadvantageously combined with a condensation of the water (i.e. waterquench). It is also advantageous to follow the desorption step with awater quench in order to minimize the losses of absorbent.

[0026] Frequently, step (c) is carried out directly after step (b), i.e.without process steps in between or intermediate stages. However, afterstep (b) and before step (c) absorbent can be separated, for example bya water quench.

[0027] The oxidation of propene and/or propane to acrolein and/oracrylic acid, which is carried out in step (c), can be effectedaccording to all processes which are known to a person skilled in theart and are not subject to any restrictions. In step (c), a one-stage ortwo-stage oxidation of propene to acrolein and/or acrylic acid or anoxidation of propane to acrolein and/or acrylic acid or both, i.e.simultaneous oxidation of propane and propene to acrolein and/or acrylicacid, can be carried out. The oxidation is expediently carried out as aselective, heterogeneously catalyzed gas-phase oxidation with molecularoxygen to give an acrolein- and/or acrylic acid-containing product gasmixture. If required, the propane and/or propene fed to the oxidation isbrought beforehand, by indirect heat exchange, to the reactiontemperature required for the oxidation reaction.

[0028] In a preferred embodiment, step (c) of the novel process iscarried out as an oxidation of propene to acrolein and/or acrylic acid.

[0029] In principle, the heterogeneously catalyzed gas-phase oxidationof propene to acrylic acid with molecular oxygen takes place in twosteps in succession along the reaction coordinate, the first of whichleads to acrolein and the second from acrolein to acrylic acid. Thisreaction sequence in two successive steps makes it possible, in a mannerknown per se, to carry out the step (c) of the novel process in twooxidation zones arranged one behind the other, it being possible for theoxidic catalyst to be used to be adapted in an optimum manner in each ofthe two oxidation zones. Thus, as a rule a catalyst based on multimetaloxides containing the combination of elements Mo—Bi—Fe is preferred forthe first oxidation zone (propene→acrolein), while catalysts based onmultimetal oxides containing the combination of elements Mo—B areusually preferred for the second oxidation zone (acrolein→acrylic acid).Corresponding multimetal oxide catalysts for the two oxidation zoneshave been widely described and are well known to a person skilled in theart. For example, EP-A-0 253 409 refers to corresponding U.S. patents onpage 5. Advantageous catalysts for the two oxidation zones are alsodisclosed in DE-A 44 31 957 and DE-A 44 31 949. This applies inparticular to those of the formula I in the two abovementionedpublications. As a rule, the product mixture from the first oxidationzone is transferred without intermediate treatment into the secondoxidation zone.

[0030] The simplest form for realizing the two oxidation zones istherefore a tube-bundle reactor, within which the catalyst load changescorrespondingly along the individual catalyst tubes with the end of thefirst reaction step. Such oxidations are described, for example, inEP-A-0 911 313, EP-A-0 979 813, EP-A-0 990 636 and DE-A 28 30 765). Ifrequired, the catalyst load in the catalyst tubes is interrupted by aninert bed.

[0031] Preferably, however, the two oxidation zones are realized in theform of two tube-bundle systems connected in series. These may bepresent in a reactor, the transition from one tube bundle to anothertube bundle being formed by a bed of inert material not housed in thecatalyst tube and expediently accessible. While the heat-transfer mediumgenerally flows around the catalyst tubes, said heat-transfer mediumdoes not reach an inert bed installed as described above. The twocatalyst tube bundles are therefore advantageously housed in reactorsspatially separated from one another. As a rule, an intermediatecondenser is present between the two tube-bundle reactors in order toreduce any subsequent acrolein combustion in the product gas mixturewhich leaves the first oxidation zone. Instead of tube-bundle reactors,plate-type heat exchanger reactors with salt cooling and/or evaporativecooling, as described, for example, in DE-A 199 29 487 and DE-A 199 52964, can also be used.

[0032] The reaction temperature in the first oxidation zone is as a rulefrom 300 to 450° C., preferably from 320 to 390° C. The reactiontemperature in the second oxidation zone is as a rule from 200 to 300°C., frequently from 220 to 290° C. The reaction pressure in bothoxidation zones is expediently from 0.5 to 5, advantageously from 1 to3, atm. The gas loading (l(S.T.P.)/l·h) of the oxidation catalysts withreaction gas is frequently from 1500 to 2500 h⁻¹ or up to 4000 h⁻¹ inboth oxidation zones. The propene loading (l(S.T.P.)/l·h) is frequentlyfrom 50 to 300 h⁻¹, in particular from 100 to 200 h⁻¹.

[0033] In principle, the two oxidation zones can be designed asdescribed, for example, in DE-A 198 37 517, DE-A 199 10 506, DE-A 199 10508 and DE-A 198 37 519. Usually, the external heating in the twooxidation zones, if desired in multizone reactor systems, is adapted ina manner known per se to the specific composition of the reaction gasmixture and to the catalyst load.

[0034] According to the invention, it is advantageous if, in the novelprocess, propene-accompanying propane acts as an advantageous inertdiluent gas in a heterogeneously catalyzed propene oxidation.

[0035] The total molecular oxygen required for the oxidation can beadded beforehand to the gas B in its total amount. However, oxygen mayalso be added to said gas after the first oxidation zone.

[0036] Preferably, a molar propene:molecular oxygen ratio of from 1:1 to1:3, frequently from 1:1.5 to 1:2, is established in the first oxidationzone. In the second oxidation zone, a molar acrolein: molecular oxygenratio of from 1:0.5 to 1:2 is preferably established.

[0037] In both oxidation zones, an excess of molecular oxygen generallyhas an advantageous effect on the kinetics of the gas-phase oxidation.Since the heterogeneously catalyzed gas-phase oxidation of the propeneto acrylic acid is subject to kinetic control, the propene can inprinciple therefore be initially taken in a molar excess relative to themolecular oxygen, for example also in the first oxidation zone. In thiscase, the excess propene factually plays the role of a diluent gas.

[0038] In principle, the heterogeneously catalyzed gas-phase oxidationof propene to acrylic acid can however also be realized in a singleoxidation zone. In this case, the two reaction steps take place in anoxidation reactor which is loaded with a catalyst which is capable ofcatalyzing the reaction of both reaction steps. Here, the catalyst loadcan also change continuously or abruptly along the reaction coordinatewithin the oxidation zone. In an embodiment of step (c) in the form oftwo oxidation zones connected in series, it is also possible partly orcompletely to separate, from the product gas mixture leaving the firstoxidation zone, oxides of carbon and steam contained in said mixture andformed as byproduct in the first oxidation zone, if required beforefurther passage into the second oxidation zone. Preferably, a procedurewhich does not require such a separation is chosen.

[0039] Suitable sources for the molecular oxygen required in theoxidation step (c) are both pure molecular oxygen and molecular oxygendiluted with inert gas, such as carbon dioxide, carbon monoxide, noblegases, nitrogen and/or saturated hydrocarbons.

[0040] In an expedient manner, air is used as an oxygen source at leastfor covering part of the requirement of molecular oxygen. The gas B fedto the oxidation step (c) of the novel process advantageously comprisessubstantially only propane and propene, and exclusively air is used as asource of molecular oxygen for the oxidation. If required, cooling ofthe gas B fed to step (c) can also be effected in a direct manner bymetering in cold air.

[0041] If acrolein is the desired product, the second oxidation zone isexpediently not used in step (c).

[0042] The oxidation of propene to acrolein and/or acrylic acid in step(c) can also be carried out as described in EP-A-0 117 146, U.S. Pat.No. 5,198,578 and U.S. Pat. No. 5,183,936 or analogously to DE-A 33 13573, CA-A-1 217 502, U.S. Pat. No. 3,161,670, U.S. Pat. No. 4,532,365and WO 97/36849. Suitable processes are also described in EP-A-0 293224, EP-A-0 253 409, DE-A 44 31 957, DE 195 08 532 or DE-A 41 32 263,particularly preferred processes being those which operate with diluentgases in the oxidation.

[0043] The oxidation of acrolein to acrylic acid can be carried out asdescribed in WO 00/39065, by means of a fluidized-bed reactor.

[0044] The oxidation of propene to acrolein and/or acrylic acid can alsobe carried out using the plate-type heat exchanger reactors described inDE-A 199 52 964.

[0045] In a further preferred embodiment, step (c) of the novel processis carried out as an oxidation of propane to acrolein and/or acrylicacid. In this oxidation, propane is converted over a suitable catalyst,in one or more stages, to acrolein and/or acrylic acid. All processesknown to a person skilled in the art are suitable for this purpose. Asuitable process is described, for example, in JP-A-10 36 311.

[0046] Catalysts suitable for the heterogeneously catalyzed gas-phaseoxidation of propane to acrolein and/or acrylic acid are multimetaloxide materials of the formula (I)

MoV_(b)M¹ _(c)M² _(d)O_(n)  (I)

[0047] where

[0048] M¹ is Te and/or Sb,

[0049] M² is at least one of the elements from the group consisting ofNb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi,B, Ce, Sn, Zn, Si and In,

[0050] b is from 0.01 to 1,

[0051] c is from >0 to 1, preferably from 0.01 to 1,

[0052] d is from >0 to 1, preferably from 0.01 to 1, and

[0053] n is a number which is determined by the valency and frequency ofthe elements other than oxygen in (I).

[0054] Multimetal oxide materials which have stoichiometry correspondingto the formula (I) are known, cf for example EP-A-0 608 838, EP-A-0 529853, JP-A 7-232 071, JP-A 10-57813, JP-A 2000-37623, JP-A 10-36311, WO00/29105, Proceedings ISO 99, Rimini (Italy), Sep. 10-11, 1999, G. Centiand S. Perathoner Ed., SCI Pub. 1999, EP-A-0 767 164, Catalysis Today 49(1999), 141-153, EP-A-0 962 253, Applied Catalysis A: General 194 to 195(2000), 479 to 485, JP-A 11/169716, EP-A-0 895 809, DE-A 198 35 247,JP-A 857319, JP-A 10-28862, JP-A 11-43314, JP-A 11-57479, WO 00/29106,JP-A 10-330343, JP-A 11-285637, JP-A 10-310539, JP-A 11-42434, JP-A11-343261, JP-A 11-343262, WO 99/03825, JP-A 7-53448, JP-A 2000-51693and JP-A 11-263745.

[0055] The multimetal oxides (I), (II) and (III) described below areparticularly suitable.

[0056] In the multimetal oxide materials (I) of the formula (I), M¹ isTe and/or Sb; M² is at least one of the elements from the groupconsisting of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt,Bi, B and Ce; b is from 0.01 to 1; c is from 0.01 to 1; d is from 0.01to 1; and n is a number which is determined by the valency and frequencyof the elements other than oxygen in (I).

[0057] The preparation of a multimetal oxide material (I) is preferablycarried out by a procedure in which a mixture of sources of theelemental constituents of the multimetal oxide material (I) is subjectedto hydrothermal treatment and the freshly forming solid is separated offand is converted into an active oxide by thermal treatment. In themultimetal oxide material (I), M¹ is preferably Te, M² is preferably Nb,b is preferably 0.1-0.6, c is preferably 0.05-0.4 and d is preferably0.01-0.6. The thermal treatment is preferably carried out at from 350 to700° C., the thermal treatment initially being effected in particular atfrom 150 to 400° C. under an oxygen-containing atmosphere and then atfrom 350 to 700° C. under an inert gas atmosphere. Suitablestoichiometries for the multimetal oxide materials (I) are those whichare disclosed in EP-A-0 608 838, WO 00/29106, JP-A 11/169716 and EP-A-0962 253.

[0058] The hydrothermal preparation of multimetal oxide active materialprecursors is familiar to a person skilled in the art (cf. for exampleApplied Catalysis A: 194 to 195 (2000), 479-485, Kinetics and Catalysis,40, No. 3 (1999), 401 to 404, Chem. Commun. (1999), 517 to 518, JP-A6/227819 and JP-A 2000/26123).

[0059] What is understood here in particular is the thermal treatment ofa preferably intimate mixture of sources of the elemental constituentsof the desired multimetal oxide material (I) in a high-pressure vessel(autoclave) in the presence of steam at superatmospheric pressure,usually at from >100 to 600° C. The pressure range is typically up to500, preferably up to 250, atm. Temperatures above 600° C. and steampressures above 500 atm may also be used, but this is not very expedientin terms of application technology. The hydrothermal treatment isparticularly advantageously carried out under conditions under whichsteam and liquid water coexist. This is possible in a temperature rangefrom >100 to 374.15° C. (critical temperature of water) using thecorresponding pressures. The amounts of water are expediently such thatthe liquid phase is capable of holding the total amount of the startingcompounds in suspension and/or solution.

[0060] However, a procedure in which the intimate mixture of thestarting compounds completely absorbs the amount of liquid water presentin equilibrium with the steam is also possible.

[0061] Advantageously, the hydrothermal treatment is carried out atfrom >100 to 300° C., preferably from 150 to 250° C. (for example from160 to 180° C.). Based on the sum of water and sources of the elementalconstituents of the desired multimetal oxide material (I), the amount ofthe latter in the autoclave is as a rule at least 1% by weight. Usually,the abovementioned amount is not above 90% by weight. Amounts of from 3to 60 or from 5 to 30, frequently from 5 to 15, % by weight are typical.

[0062] During the hydrothermal treatment, stirring may or may not beeffected. Particularly suitable starting compounds (sources) for thehydrothermal preparation variant are all those which are capable offorming oxides and/or hydroxides on heating with water undersuperatmospheric pressure. Oxides and/or hydroxides of the elementalconstituents may also be concomitantly or exclusively used as startingcompounds for the hydrothermal preparation. As a rule, the sources areused in finely divided form.

[0063] Suitable sources for the elemental constituents are all thosewhich are capable of forming oxides and/or hydroxides on heating (ifnecessary in air). Oxides and/or hydroxides of the elementalconstituents may be concomitantly or exclusively used as such startingcompounds.

[0064] Suitable sources of the element Mo are, for example, molybdenumoxides, such as molybdenum trioxide, molybdates, such as ammoniumheptamolybdate tetrahydrate and molybdenum halides, such as molybdenumchloride.

[0065] Suitable starting compounds to be used concomitantly with theelement V are, for example, vanadyl acetylacetonate, vanadates, such asammonium metavanadate, vanadium oxides, such as vanadium pentoxide(V₂O₅), vanadium halides, such as vanadium tetrachloride (VCl₄), andvanadium oxyhalides, such as VOCl₃. Expediently, vanadium startingcompounds which are concomitantly used are those which contain thevanadium in oxidation state +4.

[0066] Suitable sources for the element tellurium are tellurium oxides,such as tellurium dioxide, metallic tellurium, tellurium halides, suchas TeCl₂, and telluric acids, such as orthotelluric acid H₆TeO₆.

[0067] Advantageous antimony starting compounds are antimony halides,such as SbCl₃, antimony oxides, such as antimony trioxide (Sb₂O₃),antimonic acids, such as HSb(OH)₆, and antimony oxide salts, such asantimony oxide sulfate (SbO₂)SO₄.

[0068] Suitable niobium sources are, for example, niobium oxides, suchas niobium pentoxide (Nb₂O₅), niobium oxyhalides, such as NbOCl₃,niobium halides, such as NbCl₅, and complex compounds of niobium andorganic carboxylic acids and/or dicarboxylic acids, for example oxalatesand alcoholates. The Nb-containing solutions used in EP-A-0 895 809 arealso suitable as a niobium source.

[0069] Regarding all other possible elements M², particularly suitablestarting compounds are their halides, nitrates, formates, oxalates,acetates, carbonates and/or hydroxides. Suitable starting compounds areoften also their oxo compounds, for example tungstates, or the acidsderived from these. Frequently, ammonium salts are also used as startingcompounds.

[0070] Other suitable starting compounds are polyanions of the Andersontype, as have been described, for example, in Polyhedron 6, No. 2(1987), 213-218, and have been used, for example, in Applied CatalysisA: General 194-195 (2000), 479-485, for the preparation of suitablemultimetal oxides (I) or are disclosed in the secondary literature citedtherein. A further suitable literature source for polyanions of theAnderson type is Kinetics and Catalysis, 40 (1999), 401 to 404.

[0071] Further polyanions suitable as starting compounds are, forexample, those of the Dawson or Keggin type. Preferably used startingcompounds are those which are converted into their oxides at elevatedtemperatures either in the presence or in the absence of oxygen,possibly with liberation of gaseous compounds.

[0072] The hydrothermal treatment itself takes, as a rule, a period offrom a few hours to a few days. A period of 48 hours is typical. It isexpedient in terms of application technology if the autoclave to be usedfor the hydrothermal treatment is coated on the inside with Teflon.Before the hydrothermal treatment, the autoclave, if required includingthe aqueous mixture contained, may be evacuated. It can then be filledwith inert gas (N2; noble gas) before the temperature is increased. Bothmeasures may also be omitted. The aqaeous mixture may additionally beflushed with inert gas for blanketing prior to the hydrothermaltreatment. It is also expedient in terms of application technology ifthe abovementioned inert gases are used for establishingsuperatmospheric pressure in the autoclave before the hydrothermaltreatment.

[0073] The required treatment of the solid freshly formed in the courseof the hydrothermal treatment and separated off after the end of thehydrothermal treatment (after the end of the hydrothermal treatment, theautoclave can be either quenched to room temperature or brought to roomtemperature slowly, i.e. over a relatively long period (for example byleaving it to stand)) is expediently carried out at from 350 to 700° C.,frequently from 400 to 650° C. or from 400 to 600° C. It can be effectedunder an oxidizing, reducing or inert atmosphere. A suitable oxidizingatmosphere is, for example, air, air enriched with molecular oxygen orair depleted in oxygen. Preferably, the thermal treatment is carried outunder an inert atmosphere, i.e. for example under molecular nitrogenand/or noble gas. Of course, the thermal treatment can also be effectedunder reduced pressure.

[0074] If the thermal treatment is carried out under a gaseousatmosphere, this may be either stationary or flowing.

[0075] In general, the thermal treatment may take up to 24 hours ormore.

[0076] The thermal treatment is preferably carried out initially underan oxidizing (oxygen-containing) atmosphere (for example under air) atfrom 150 to 400° C. or from 250 to 350° C. The thermal treatment is thenexpediently continued under inert gas at from 350 to 700° C. or from 400to 650° C. or from 400 to 600° C. The thermal treatment of thehydrothermally produced catalyst precursor can also be effected in sucha way that the catalyst precursor material is first pelleted, thenthermally treated and subsequently converted into chips.

[0077] It is expedient in terms of application technology if the solidobtained in the hydrothermal treatment is converted into chips for thesubsequent thermal treatment.

[0078] If the starting compounds used for the preparation of themultimetal oxide materials (I) are the same as those used for aconventional preparation of multimetal oxides (I) and the thermaltreatment of the conventionally produced intimate dry blend is carriedout in the same way as the thermal treatment of the hydrothermallyobtained solid, the multimetal oxide materials (I) generally have ahigher selectivity of the acrylic acid formation and a higher activitywith respect to the heterogeneously catalyzed gas-phase oxidation ofpropane to acrylic acid under the same conditions.

[0079] The multimetal oxide materials (I) can be used as such (forexample after comminution to a powder or to chips) or can be convertedinto moldings before being used. The catalyst bed may be either a fixedbed, a moving bed or a fluidized bed.

[0080] The X-ray diffraction pattern of the multimetal oxide materials(I) corresponds as a rule essentially to those in EP-A-0 529 853, DE-A198 35 247 and EP-A-0 608 838.

[0081] The multimetal oxide materials (I) can also be used in a formdiluted with finely divided, for example colloidal, materials, such assilica, titanium dioxide, alumina, zirconium oxide or niobium oxide.

[0082] The mass dilution ratio may be up to 9 (diluent): 1 (activematerial). This means that possible mass dilution ratios are also 6(diluent): 1 (active material) and 3 (diluent): 1 (active material). Thediluent can be incorporated before and/or after the calcination. As arule, the diluent is incorporated before the hydrothermal treatment. Ifthe incorporation is effected before the calcination, the diluent mustbe chosen so that it is substantially retained as such during thecalcination. The same applies to the hydrothermal treatment in the caseof incorporation before said treatment is carried out. As a rule, thisis true, for example, in the case of oxides calcined at correspondinglyhigh temperatures.

[0083] Other catalysts suitable for the propane oxidation are multimetaloxide materials (II) which have the abovementioned formula (I) and whoseX-ray diffraction pattern has reflections h, i and k whose peaks are atthe diffraction angles (2θ) 22.2±0.4° (h), 27.3±0.4° (i) and 28.2±0.4°(k), where

[0084] the reflection h is the one with the highest intensity within theX-ray diffraction pattern and has a half-width of not more than 0.5°,

[0085] the intensity Pi of the reflection i and the intensity P_(k) ofthe reflection k fulfill the relationship 0.65≦R≦0.85, in which R is theintensity ratio defined by the formula

R=P_(i)/(P_(i)+P_(k)),

[0086] and

[0087] the half-width of the reflection i and of the reflection k is ≦1°in each case.

[0088] Preferably, 0.67 <R<0.75 and very particularly preferably R=0.70to 0.75 or R 0.72.

[0089] The use of multimetal oxide materials (II) where M¹ is Te ispreferred. Furthermore, those multimetal oxide materials (II) in whichM² is Nb, Ta, W and/or Ti are advantageous. Preferably, M² is Nb.

[0090] The stoichiometric coefficient b of the multimetal oxidematerials (II) is advantageously from 0.1 to 0.6. In a correspondingmanner, the preferred range for the stoichiometric coefficient c is from0.01 to 1 or from 0.05 to 0.4 and advantageous values for d are from0.01 to 1 or from 0.1 to 0.6. Particularly advantageous multimetal oxidematerials (II) are those in which the stoichiometric coefficients b, cand d are simultaneously in the abovementioned preferred ranges. Furthersuitable stoichiometries for the multimetal oxide materials (II) arethose which are disclosed in the publications of the prior art citedabove, in particular those disclosed in JP-A 7-53448.

[0091] A specific process for the preparation of multimetal oxidematerials (II) is disclosed, for example, in JP-A 11-43314, in which therelevant multimetal oxide materials (II) are recommended as catalystsfor the heterogeneously catalyzed oxydehydrogenation of ethane toethene.

[0092] Thereafter, a multimetal oxide material of the formula (I) whichis a mixture of the i-phase and other phases (for example k-phase) isfirst produced in a manner known per se and disclosed in the cited priorart publications. In this mixture, for example, the proportion ofi-phase can be increased by selectively removing the other phases, forexample the k-phase, under the microscope or washing the multimetaloxide material with suitable liquids. Suitable such liquids are, forexample, aqueous solutions of organic acids (for example oxalic acid,formic acid, acetic acid, citric acid and tartaric acid), inorganicacids (for example nitric acid), alcohols and aqueous hydrogen peroxidesolutions. Furthermore, JP-A 7-232071 discloses a process for thepreparation of multimetal oxide materials (II).

[0093] Multimetal oxide materials (II) are obtainable by the preparationmethod according to DE-A 198 35 247. According to this, a very intimate,preferably finely divided, dry blend is produced from suitable sourcesof their elemental constituents and said dry blend is subjected to athermal treatment at from 350 to 700° C. or from 400 to 650° C. or from400 to 600° C. The thermal treatment can be carried out under either anoxidizing, reducing or inert atmosphere. A suitable oxidizing atmosphereis, for example, air, air enriched with molecular oxygen or air depletedin oxygen. Preferably, the thermal treatment is carried out under aninert atmosphere, i.e. for example under molecular nitrogen and/or noblegas.

[0094] Usually, the thermal treatment is effected at atmosphericpressure (1 atm). Of course, the thermal treatment can also be effectedunder reduced or superatmospheric pressure.

[0095] If the thermal treatment is carried out under a gaseousatmosphere, this may be either stationary or flowing. In general, thethermal treatment may take up to 24 hours or more.

[0096] The thermal treatment is initially preferably carried out underan oxidizing (oxygen-containing) atmosphere (for example under air) atfrom 150 to 400° C. or from 250 to 350° C. Thereafter, the thermaltreatment is expediently continued under an inert gas at from 350 to700° C. or from 400 to 650° C. or from 400 to 600° C. The thermaltreatment can also be effected in such a way that the catalyst precursormaterial is first pelleted (if required after pulverization and, ifrequired, with the addition of from 0.5 to 2% by weight of finelydivided graphite) before its thermal treatment, then subjected to thethermal treatment and subsequently converted into chips.

[0097] The thorough mixing of the starting compounds in the preparationof the multimetal oxide materials (II) can be effected in dry or in wetform. If it is effected in dry form, the starting compounds areexpediently used in the form of finely divided powders and, after mixingand any compaction, are subjected to the calcination (thermaltreatment). However, the thorough mixing is preferably effected in wetform. Usually, the starting compounds are mixed with one another in theform of an aqueous solution and/or suspension. Thereafter, the aqueousmaterial is dried and is calcined after the drying. Expediently, theaqueous material is an aqueous solution or an aqueous suspension.Preferably, the drying process is carried out immediately after thepreparation of the aqueous mixture and by spray-drying (the outlettemperatures are as a rule from 100 to 150° C.; the spray-drying can becarried out by the cocurrent or countercurrent method), which requires aparticularly intimate dry blend, especially when the aqueous material tobe spray-dried is an aqueous solution or suspension.

[0098] Suitable sources or starting compounds for the multimetal oxidematerial (II) are the compounds described above in the case of themultimetal oxide material (I).

[0099] The multimetal oxide materials (II) can be converted intomoldings as in the case of the multimetal oxide materials (I) and can beused in the same way as these. The shaping of the multimetal oxidematerials (II) can be effected, for example, by application to asupport, as described below under catalyst (III), or by extrusion and/orpelleting, both of finely divided multimetal oxide material (II) and offinely divided precursor material of a multimetal oxide material (II).

[0100] In the same way as the multimetal oxide materials (I), themultimetal oxide materials (II) can also be used in a form diluted withfinely divided materials.

[0101] Suitable geometries are spheres, solid cylinders and hollowcylinders (rings). The longest dimension of the abovementionedgeometries is as a rule from 1 to 10 mm. In the case of cylinders, theirlength is preferably from 2 to 10 mm and their external diameterpreferably from 4 to 10 mm. In the case of rings, the wall thickness ismoreover usually from 1 to 4 mm. Suitable annular unsupported catalystsmay also have a length from 3 to 6 mm, an external diameter of from 4 to8 mm and a wall thickness of from 1 to 2 mm. However, an annularunsupported catalyst having the dimensions of 7 mm×3 mm×4 mm or of 5mm×3 mm×2 mm (external diameter x length x internal diameter) is alsopossible.

[0102] The definition of the intensity of a reflection in the X-raydiffraction pattern is based here on the definition stated in DE-A 19835 247 and that in DE-A 100 51 419 and DE-A 100 46 672.

[0103] This means that if A¹ is the peak of a reflection 1 and B¹ is thenext pronounced minimum (minima having the reflection shoulders are nottaken into account) to the left of peak A¹ in the line of the X-raydiffraction pattern when viewed along the intensity axis perpendicularto the 2θ axis and B² is correspondingly the next pronounced minimum tothe right of the peak A¹ and C¹ is a point at which a straight linedrawn from the peak A¹ perpendicular to the 2θ axis intersects astraight line connecting the points B¹ and B², then the intensity ofreflection 1 is the length of the straight line section A¹C¹ whichextends from the peak A¹ to the point C¹. The expression minimum means apoint at which the slope of a tangent to the curve in a base region ofreflection 1 changes from a negative value to a positive value, or apoint at which the slope tends to zero, the coordinates of the 2θ axisand of the intensity axis being used for specifying the slope.

[0104] Here, the half-width is correspondingly the length of thestraight line section which is present between the two points ofintersection H¹ and H² if a parallel to the 2θ axis is drawn in thecenter of the straight line section A¹C¹, H¹ and H² being in each casethe first point of intersection of this parallel, to the left and rightof A¹, with that line of the X-ray diffraction pattern which is definedas above.

[0105] An exemplary procedure for the determination of half-width andintensity is also shown in FIG. 6 in DE-A 100 46 672.

[0106] In addition to the reflections h, i and k, the X-ray diffractionpattern of advantageous catalytically active multimetal oxide materials(II) contains, as a rule, further reflections whose peaks are at thefollowing diffraction angles (2θ):

[0107] 9.0±0.4° (l),

[0108] 6.7±0.4° (o) and

[0109] 7.9±0.4° (p).

[0110] It is advantageous if the X-ray diffraction pattern of thecatalytically active oxide materials of the formula (I) additionallycontains a reflection whose peak is at the following diffraction angle(2θ):

[0111] 45.2±0.4° (q).

[0112] Frequently, the X-ray diffraction pattern of the multimetal oxidematerials (II) also contains the reflections 29.2±0.4° (m) and 35.4±0.4°(n).

[0113] The multimetal oxide material (II) may be one whose X-raydiffraction pattern has no reflection with a peak position of2θ=50.0±0.3°, i.e. one which contains no k-phase.

[0114] However, the multimetal oxide material (II) can also contain ak-phase, its X-ray diffraction pattern generally also containing furtherreflections whose peaks are at the following diffraction angles (2θ):

[0115] 36.2±0.40 and

[0116] 50.0±10.4°.

[0117] If the reflection h is assigned the intensity 100, it isadvantageous if the reflections i, l, m, n, o, p and q have thefollowing intensities on the same intensity scale:

[0118] i: from 5 to 95, frequently from 5 to 80, in some cases from 10to 60;

[0119] l: from 1 to 30;

[0120] m: from 1 to 40;

[0121] n: from 1 to 40;

[0122] o: from 1 to 30;

[0123] p: from 1 to 30 and

[0124] q: from 5 to 60.

[0125] If the X-ray diffraction pattern contains additional reflectionsfrom those stated above, the half-width thereof is as a rule <1°.

[0126] All data based here on an X-ray diffraction pattern relate to anX-ray diffraction pattern produced using Cu—Kα radiation as theX-radiation (Siemens diffractometer Theta-Theta D-5000, tube voltage: 40kV, tube current: 40 mA, aperture V20 (variable), collimator V20(variable), secondary monochromator aperture (0.1 mm), detector aperture(0.6 mm), measuring interval (2θ): 0.02°, measuring time per step: 2.4s, detector: scintillation counter).

[0127] The specific surface area of multimetal oxide materials (II) isoften from 1 to 30 m²/g (BET surface area, nitrogen).

[0128] Another suitable catalyst for the propane oxidation is a catalyst(III) which consists of a support and a catalytically active oxidematerial of the abovementioned formula (I) which is applied to thesurface of the support.

[0129] The use of oxide materials of the formula (I) where M¹ is Te ispreferred. It is furthermore advantageous if M² is Nb, Ta, W and/or Ti.Preferably, M² is Nb.

[0130] The stoichiometric coefficient b of the oxide materials of theformula (I) in catalyst (III) is advantageously from 0.1 to 0.6. In acorresponding manner, the preferred range for the stoichiometriccoefficient c is from 0.01 to 1 or from 0.05 to 0.4, and advantageousvalues for d are from 0.01 to 1 or from 0.1 to 0.6. Particularlyadvantageous oxide materials are those in which the stoichiometriccoefficients b, c and d are simultaneously in the abovementionedpreferred ranges.

[0131] Further suitable stoichiometries for the oxide materials of theformula (I) are those which are disclosed in the abovementionedpublications, in particular those which are disclosed in EP-A-0 608 838,WO 00/29106, JP-A 11/169716 and EP-A-0 962 253.

[0132] The application of the above-described multimetal oxide material(II) as the oxide material of the formula (I) to a support is alsoparticularly preferred for the preparation of the catalyst (III).

[0133] The supports are preferably chemically inert, i.e. they playsubstantially no part in the course of the catalytic gas-phase oxidationof propane to acrylic acid. Particularly suitable materials for thesupports are alumina, silica, silicates, such as clay, kaolin, steatite,pumice, aluminum silicate and magnesium silicate, silicon carbide,zirconium dioxide and thorium dioxide.

[0134] The surface of the support may be either smooth or rough.Advantageously, the surface of the support is rough since increasedsurface roughness generally results in high adhesive strength of theapplied active material coat.

[0135] Frequently, the surface roughness Rz of the support is from 5 to200 μm, often from 20 to 100 μm (determined according to DIN 4768, Sheet1, using a Hommel tester for DIN-ISO measured surface variables fromHommelwerke, Germany).

[0136] Furthermore, the support material may be porous or nonporous. Thesupport material is expediently nonporous (total volume of the pores ≦1%by volume, based on the volume of the support).

[0137] The thickness of the active oxide material coat present on thecoated catalyst is usually from 10 to 1 000 mm. However, it may also befrom 50 to 700 μm, from 100 to 600 μm or from 300 to 500 μm or from 150to 400 μm. Possible coated thicknesses are also from 10 to 500 μm, from100 to 500 μm or from 150 to 300 μm.

[0138] In principle, any desired geometries of the supports aresuitable. Their longest dimension is as a rule from 1 to 10 mm. However,spheres or cylinders, in particular hollow cylinders (rings), arepreferably used as supports. Advantageous diameters of the supportspheres are from 1.5 to 4 mm. If cylinders are used as supports, theirlength is preferably from 2 to 10 mm and their external diameterpreferably from 4 to 10 mm. In the case of rings, the wall thickness ismoreover usually from 1 to 4 mm. Suitable annular supports can also havea length of from 3 to 6 mm, an external diameter of from 4 to 8 mm and awall thickness of from 1 to 2 mm. However, an annular support havingmeasurements of 7 mm×3 mm×4 mm or of 5 mm×3 mm×2 mm (external diameter xlength x internal diameter) is also possible.

[0139] The preparation of the catalysts (III) can be carried out in avery simple manner by preforming active oxide materials of the formula(I), converting them into a finely divided form and finally applyingthem to the surface of the support with the aid of a liquid binder. Forthis purpose, the surface of the support is moistened in a very simplemanner with the liquid binder and a coat of the active material iscaused to adhere to the moistened surface by bringing said surface intocontact with finely divided active oxide materials of the formula (I).Finally, the coated support is dried. Of course, the process can berepeated periodically to achieve a thicker coat. In this case, thecoated parent structure becomes the new support, etc.

[0140] The fineness of the catalytically active oxide material of theformula (I) which is to be applied to the surface of the support isadapted to the desired coat thickness. For example, those activematerial powders of which at least 50% of the total number of powderparticles pass through a sieve of mesh size of from 1 to 20 μm and whosenumerical proportion of particles having a longest dimension above 50 μmis less than 10% are suitable for the coat thickness range of from 100to 500 μm. As a rule, the distribution of the longest dimensions of thepowder particles corresponds to a Gaussian distribution as a result ofthe preparation.

[0141] For carrying out the described coating process on an industrialscale, it is advisable, for example, to use the process principledisclosed in DE-A 29 096 71. There, the supports to be coated areinitially taken in a preferably inclined (the angle of inclination is asa rule ≧0° and ≦90°, in general ≧30° and ≦90°; the angle of inclinationis the angle of the central axis of the rotating container relative tothe horizontal) rotating container (for example rotating pan or coatingdrum). The rotating container transports the supports, which for exampleare spherical or cylindrical, under two metering apparatuses arranged aspecific distance apart. The first of the two metering apparatusesexpediently corresponds to a nozzle (for example an atomizer nozzleoperated with compressed air) through which the supports rolling in therotating pan are sprayed and moistened in a controlled manner with theliquid binder. The second metering apparatus is located outside theatomization cone of the liquid binder sprayed in and serves for feedingin the finely divided oxidic active material (for example via a shakingconveyor or a powder screw). The support spheres moistened in acontrolled manner take up the supplied active material powder which, asa result of the rolling movement, becomes compacted to a cohesive coaton the outer surface of the support, which for example is cylindrical orspherical.

[0142] If required, the support provided with the basecoat in thismanner passes through the spray nozzles again in the course of thesubsequent revolution, is moistened in a controlled manner in order tobe able to take up a further coat of finely divided oxidic activematerial in the course of the further movement, etc. (intermediatedrying is as a rule not necessary). Finely divided oxidic acid materialand liquid binder are as a rule fed in continuously and simultaneously.

[0143] The liquid binder can be removed after the end of the coating,for example by the action of hot gases, such as N₂ or air. The coatingprocess described is known to provide satisfactory adhesion both of thesuccessive coats to one another and of the basecoat to the surface ofthe support.

[0144] What is important for the coating procedure described above isthat the moistening of the support surface to be coated is carried outin a controlled manner. Briefly, this means that the support surface isexpediently moistened so that it has adsorbed liquid binder but noliquid phase as such is visible on the support surface. If the supportsurface is too moist, the finely divided catalytically active oxidematerial forms separate agglomerates instead of being deposited on thesurface. Detailed information in this context is to be found in DE-A 2909 671.

[0145] The abovementioned final removal of the liquid binder used can beeffected in a controlled manner, for example by evaporation and/orsublimation. In the simplest case, this can be effected by the action ofhot gases of corresponding temperature (frequently from 50 to 300° C.,often 150° C.). However, only preliminary drying can be effected by theaction of hot gases. The final drying can then be carried out, forexample, in a drying oven of any desired type (for example a belt dryer)or in the reactor. The temperature employed should not be above thecalcination temperature used for the preparation of the oxidic activematerial. Of course, the drying can also be carried out exclusively in adrying oven.

[0146] Regardless of type and geometry of the support, the following canbe used as binders for the coating process: water, monohydric alcohols,such as ethanol, methanol, propanol and butanol, polyhydric alcohols,such as ethylene glycol, 1,4-butanediol, 1,6-hexanediol or glycerol,monobasic or polybasic organic carboxylic acids, such as propionic acid,oxalic acid, malonic acid, glutaric acid or maleic acid, aminoalcohols,such as ethanolamine or diethanolamine, and monofunctional orpolyfunctional organic amides, such as formamide. Advantageous bindersare also solutions consisting of from 20 to 90% by weight of water andfrom 10 to 80% by weight of an organic compound which is dissolved inwater and whose boiling point or sublimation temperature at atmosphericpressure (1 atm) is >100° C., preferably >150° C. The organic compoundis advantageously selected from the above list of possible organicbinders. Preferably, the organic fraction of the abovementioned aqueousbinder solutions is from 10 to 50, particularly preferably from 20 to30, % by weight. Other suitable organic components are monosaccharidesand oligosaccharides, such as glucose, fructose, sucrose or lactose, andpolyethylene oxides and polyacrylates.

[0147] The preparation of the catalytically active oxide materials ofthe formula (I) can be carried out in a manner known per se, as in theprior art publications cited above, i.e. the preparation can be carriedout, for example, both hydrothermally and in a conventional manner.

[0148] In the latter case, the catalytically active oxide materials ofthe formula (I) are obtainable by producing from suitable sources oftheir elemental constituents a very intimate, preferably finely divideddry blend and subjecting the latter to a thermal treatment at from 350to 700° C. or from 400 to 650° C. or from 400 to 600° C. The thermaltreatment can be effected as described above in the case of themultimetal oxide material (II). The thorough mixing of the startingcompounds can also be carried out as described above in the case ofmultimetal oxide material (II).

[0149] Suitable sources of the elemental constituents when carrying outthe above-described preparation procedure for the catalytically activeoxide materials of the formula (I) are the starting compounds or sourcesdescribed above in the case of the multimetal oxide material (I).

[0150] Coated catalysts which have the multimetal oxide material (II) ascatalytically active oxide material of the formula (I) are particularlypreferred.

[0151] However, the active oxide materials of the formula (I) from WO00/29106, which substantially have an amorphous structure which appearsin the X-ray diffraction pattern in the form of very broad reflectionshaving peaks at the 2θ angles of about 22° and about 27°, are alsosuitable for producing the coated catalysts.

[0152] However, the active oxide materials of the formula (I) fromEP-A-0 529 853 and from EPA-0 608 838, which have very narrowreflections at 2θ peak positions of 22.1±0.3°, 28.2±0.3°, 36.2±0.3°,45.2±0.3° and 50.0±0.3° in the X-ray diffraction pattern, are alsosuitable.

[0153] The coated catalysts can be prepared not only by applying thefinished, finely milled active oxide materials of the formula (I) to themoistened support surface; instead of the active oxide material, afinely divided precursor material thereof can also be applied to themoistened support surface (using the same coating process and binder)and the calcination can be carried out after drying of the coatedsupport. A suitable finely divided precursor material of this type is,for example, the material which is obtainable by first producing, fromthe sources of the elemental constituents of the desired active oxidematerial of the formula (I), a very intimate, preferably finely divided,dry blend (for example by spray-drying an aqueous suspension or solutionof the sources) and subjecting this finely divided dry blend (ifnecessary after pelleting with addition of from 0.5 to 2% by weight offinely divided graphite) to a thermal treatment for a few hours at from150 to 350° C., preferably from 250 to 350° C. under an oxidizing(oxygen-containing) atmosphere (for example under air) and, if required,finally subjecting it to milling. After the coating of the supports withthe precursor material, calcination is then effected, preferably underan inert gas atmosphere (all other atmospheres are also suitable) atfrom 360 to 700° C. or from 400 to 650° C. or from 400 to 600° C.

[0154] The multimetal oxide materials (II) described above or thecatalysts (III) comprising the multimetal oxide material (II) as thecatalytically active oxide material can also be used for the oxidationof propene. Here, the propene can be oxidized in the presence ofpropane. If propane is used as a diluent gas, some of it can also beoxidized to acrylic acid.

[0155] The procedure for the propane oxidation is not subject to anyrestrictions. It can be carried out according to all processes known toa person skilled in the art. For example, the procedure described inEP-A-0 608 838 or WO 00/29106 can be employed, i.e. a gas B with whichthe catalyst is to be loaded at reaction temperatures of, for example,from 200 to 550° C. or from 230 to 480° C. or from 300 to 440° C. instep (c) may have, for example, the following composition:

[0156] from 1 to 15, preferably from 1 to 7, % by volume of propane,

[0157] from 44 to 99% by volume of air and

[0158] from 0 to 55% by volume of steam.

[0159] Other possible compositions of the gas mixture fed to step (c)for producing the gas B are:

[0160] from 70 to 95% by volume of propane,

[0161] from 5 to 30% by volume of molecular oxygen and

[0162] from 0 to 25% by volume of steam.

[0163] The plate-type heat exchanger reactors described in DE-A 199 52964 are also suitable for carrying out the propane oxidation. In anotherembodiment of the present invention, the propane oxidation is carriedout according to the processes described in DE-A 198 37 517, DE-A 198 37518, DE-A 198 37519 and DE-A 198 37 520.

[0164] The product gas mixture leaving the propene oxidation and/orpropane oxidation as step (c) of the novel process does not exclusivelyconsist of the desired product acrolein and/or acrylic acid but is as arule composed substantially of acrolein and/or acrylic acid, unconvertedmolecular oxygen, propane, propene, molecular nitrogen, steam formed asa byproduct and/or concomitantly used as diluent gas, oxides of carbonwhich are formed as a byproduct and/or concomitantly used as diluentgas, and small amounts of other lower aldehydes, hydrocarbons and otherinert diluent gases.

[0165] The desired product acrolein and/or acrylic acid can be separatedfrom the product gas mixture in a manner known per se, for example byazeotropic separation, fractional distillation (with or without asolvent) or crystallization. For example, partial condensation of theacrylic acid, absorption of acrylic acid in water or in a high-boilinghydrophobic organic solvent or absorption of acrolein in water or inaqueous solutions of lower carboxylic acids with subsequent working-upof the absorbates is suitable; alternatively, the product gas mixturecan also be subjected to fractional condensation, cf. for example EP-A-0117 146, DE-A 43 08 087, DE-A 43 35 172, DE-A 44 36 243, DE-A 199 24 532and DE-A 199 24 533.

[0166] In a particularly preferred embodiment of the novel process,after step (c) has been carried out and the desired product isolated,unreacted propane and/or propene are then separated from the remaininggas mixture according to the invention in steps (a) and (b) and arerecycled to step (c).

[0167] The gas mixture A used in step (a) of the novel process may alsobe a gas mixture which has the composition of a gas mixture which isobtainable by catalytic dehydrogenation of propane to propene. Here, thedehydrogenation can be effected by oxidation, i.e. by supplying oxygen,or without a supply of oxygen, in particular substantially without asupply of oxygen. In the dehydrogenation with a supply of oxygen, adistinction may be made between two cases. In one case, all hydrogenformed is oxidized by an excess of oxygen so that the product gas nolonger contains any hydrogen but excess oxygen (oxidativedehydrogenation). In the second case, only sufficient oxygen is added tocover the enthalpy of reaction, so that no oxygen is contained in theproduct gas but hydrogen may well be (autothermal procedure). Thepropane dehydrogenation can be carried out catalytically orhomogeneously (noncatalytically).

[0168] Dehydrogenation of propane can be carried out, for example, asdescribed in DE-A 33 13 573 and EP-A-0 117 146.

[0169] In principle, the oxidative propane dehydrogenation can becarried out as a homogeneous and/or heterogeneously catalyzedoxydehydrogenation of propane to propene with molecular oxygen. If thisfirst reaction stage is designed as a homogeneous oxydehydrogenation, itcan be carried out in principle as described, for example, in U.S. Pat.No. 3,798,283, CN-A-1 105 352, Applied Catalysis 70(2) (1991), 175-187,Catalysis Today 13 (1992), 673-678, and DE-A-196 22 331, it also beingpossible to use air as the oxygen source.

[0170] The temperature of the homogeneous oxydehydrogenation isexpediently chosen to be in the range from 300 to 700° C., preferablyfrom 400 to 600° C., particularly preferably from 400 to 500° C. Theoperating pressure may be from 0.5 to 100, in particular from 1 to 10,bar. The residence time is usually from 0.1 or 0.5 to 20, preferablyfrom 0.1 or 0.5 to 5, seconds. The reactor used may be, for example, atubular furnace or a tube-bundle reactor, for example a countercurrenttubular furnace with stack gas as a heat-transfer medium or tube-bundlereactor with salt melt as a heat-transfer medium. The propane-to-oxygenratio in the starting mixture is preferably from 0.5:1 to 40:1, inparticular from 1:1 to 6:1, more preferably from 2:1 to 5:1. Thestarting mixture may also comprise further, substantially inert,components, such as water, carbon dioxide, carbon monoxide, nitrogen,noble gases and/or propene, it also being possible for these to berecycled components. Here, components recycled to stage (a) aregenerally referred to as recycle gas.

[0171] If the propane dehydrogenation is designed as a heterogeneouslycatalyzed oxydehydrogenation, it can be carried out in principle asdescribed, for example, in U.S. Pat. No. 4,788,371, CN-A 1073893,Catalysis Letters 23 (1994), 103-106, W. Zhang, Gaodeng Xuexiao HuaxueXuebao 14 (1993), 566, Z. Huang, Shiyou Huagong 21 (1992), 592, WO97/36849, DE-A 197 53 817, U.S. Pat. No. 3,862,256, U.S. Pat. No.3,887,631, DE-A 195 30 454, U.S. Pat. No. 4,341,664, J. of Catalysis 167(1997), 560-569, J. of Catalysis 167 (1997), 550-559, Topics inCatalysis 3 (1996), 265-275, U.S. Pat. No. 5,086,032, Catalysis Letters10 (1991), 181-192, Ind. Eng. Chem. Res. 35 (1996), 14-18, U.S. Pat. No.4,255,284, Applied Catalysis A: General 100 (1993), 111-130, J. ofCatalysis 148 (1994), 56-67, V. Cortes Corberan and S. Vic Bellón (Ed.),New Developments in Selective Oxidation II, 1994, Elsevier Science B.V.,pages 305-313, 3^(rd) World Congress on Oxidation Catalysis, R. K.Grasselli, S. T. Oyama, A. M. Gaffney and J. E. Lyons (Ed.), 1997,Elsevier Science B.V., page 375 et seq. Air may also be used as theoxygen source. Preferably, however, the oxygen source comprises at least90, more preferably 95, mol %, based on 100 mol % of the oxygen source,of oxygen.

[0172] The catalysts suitable for the heterogeneous oxydehydrogenationare not subject to any particular restrictions. All oxydehydrogenationcatalysts which are known to a person skilled in the art in this areaand which are capable of oxidizing propane to propene are suitable. Inparticular, all oxydehydrogenation catalysts stated in theabovementioned publications may be used. Preferred catalysts include,for example, oxydehydrogenation catalysts which comprise MoVNb oxides orvanadyl pyrophosphate, each with a promoter. An example of aparticularly suitable catalyst is a catalyst which contains a mixedmetal oxide comprising Mo, V, Te, O and X as substantial components,where X is at least one element selected from niobium, tantalum,tungsten, titanium, aluminum, zirconium, chromium, manganese, iron,ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony,bismuth, boron, indium and cerium. Other particularly suitableoxydehydrogenation catalysts are the multimetal oxide materials ormultimetal oxide catalysts A of DE-A-197 53 817, the multimetal oxidematerials or multimetal oxide catalysts A stated in the abovementionedpublication as being preferred being very particularly advantageous.This means that particularly suitable active materials are multimetaloxide materials (IV) of the formula IV

M¹ _(a)MO_(1-b)M² _(b)O_(x)  (IV),

[0173] where

[0174] M¹ is Co, Ni, Mg, Zn, Mn and/or Cu,

[0175] M² is W, V, Te, Nb, P, Cr, Fe, Sb, Ce, Sn and/or La,

[0176] a is 0.5-1.5

[0177] b is 0-0.5

[0178] and

[0179] x is a number which is determined by the valency and frequency ofthe elements other than oxygen in (IV).

[0180] In principle, suitable active materials (IV) can be prepared in asimple manner by producing, from suitable sources of their elementalconstituents, a very intimate, preferably finely divided, dry blendhaving a composition corresponding to their stoichiometry and calciningthis dry blend at from 450 to 1 000° C. Suitable sources of theelemental constituents of the multimetal oxide active materials (IV) arethose compounds which are oxides and/or those compounds which can beconverted into oxides by heating, at least in the presence of oxygen.These are in particular halides, nitrates, formates, oxalates, citrates,acetates, carbonates, ammine complex salts, ammonium salts and/orhydroxides. The thorough mixing of the starting compounds for thepreparation of the multimetal oxide materials (IV) can be effected indry form, for example as finely divided powder, or in wet form, forexample using water as a solvent. The multimetal oxide materials (IV)can be used both in powder form and after shaping to give specificcatalyst geometries, it being possible to carry out the shaping beforeor after the final calcination. Unsupported catalysts may be used, orthe shaping of a pulverulent active material or precursor material canalso be effected by application to preshaped inert catalyst supports.Conventional, porous or nonporous aluminas, silica, thorium dioxide,zirconium dioxide, silicon carbide or silicate can be used as supportmaterials, it being possible for the supports to have a regular orirregular shape.

[0181] For the heterogeneously catalyzed oxydehydrogenation of propane,the reaction temperature is preferably from 200 to 600° C., inparticular from 250 to 500° C., more preferably from 350 to 440° C. Theoperating pressure is preferably from 0.5 to 10, in particular from 1 to10, more preferably from 1 to 5, bar. Operating pressures above 1 bar,for example from 1.5 to 10 bar, have proven particularly advantageous.As a rule, the heterogeneously catalyzed oxydehydrogenation of propaneis carried out over a fixed catalyst bed. The latter is expedientlyloaded into the tubes of a tube-bundle reactor, as described, forexample, in EP-A-0 700 893 and in EP-A-0 700 714 and the literaturecited in these publications. The average residence time of the reactiongas mixture in the catalyst bed is expediently from 0.5 to 20 seconds.The ratio of propane to oxygen varies with the desired conversion andthe selectivity of the catalyst and is expediently from 0.5:1 to 40:1,in particular from 1:1 to 6:1, more preferably from 2:1 to 5:1. As arule, the propene selectivity decreases with increasing propaneconversion. The propane-to-propene reaction is therefore preferablycarried out in such a way that relatively low propane conversions areachieved in combination with high propene selectivity. The propaneconversion is particularly preferably from 5 to 40%, more preferablyfrom 10 to 30%. Here, the term propane conversion means the proportionof supplied propane which is converted. The selectivity is particularlypreferably from 50 to 98%, more preferably from 80 to 98%, the termselectivity referring to the number of moles of propene which areproduced per mole of converted propane, expressed as a percentage.

[0182] Preferably, the starting mixture used in the oxidative propanedehydrogenation contains from 5 to 95% by weight, based on 100% byweight of starting mixture, of propane. In addition to propane andoxygen, the starting mixture for the heterogeneously catalyzedoxydehydrogenation may also comprise further, in particular inert,components, such as water, carbon dioxide, carbon monoxide, nitrogen,noble gases and/or propene. The heterogeneous oxydehydrogenation canalso be carried out in the presence of diluents, for example steam.

[0183] Any desired reactor sequence which is known to a person skilledin the art may be used for carrying out the homogeneousoxydehydrogenation or the heterogeneously catalyzed oxydehydrogenation.For example, the reaction can be carried out in a single stage or in twoor more stages between which oxygen is introduced. It is also possibleto use homogeneous and heterogeneously catalyzed oxydehydrogenations incombination with one another.

[0184] As possible constituents, the product mixture of the propaneoxydehydrogenation may contain, for example, the following components:propene, propane, carbon dioxide, carbon monoxide, water, nitrogen,oxygen, ethane, ethene, methane, acrolein, acrylic acid, ethylene oxide,butane, acetic acid, formaldehyde, formic acid, propylene oxide andbutene. A preferred product mixture obtained in the propaneoxydehydrogenation contains: from 5 to 10% by weight of propene, from 1to 2% by weight of carbon monoxide, from 1 to 3% by weight of carbondioxide, from 4 to 10% by weight of water, from 0 to 1% by weight ofnitrogen, from 0.1 to 0.5% by weight of acrolein, from 0 to 1% by weightof acrylic acid, from 0.05 to 0.2% by weight of acetic acid, from 0.01to 0.05% by weight of formaldehyde, from 1 to 5% by weight of oxygen,from 0.1 to 1.0% by weight of further abovementioned components andpropane as the remainder, based in each case on 100% by weight ofproduct mixture.

[0185] In general, the propane dehydrogenation for the preparation ofgas mixture A can also be carried out as a heterogeneously catalyzedpropane dehydrogenation substantially in the absence of oxygen, asdescribed in DE-A 33 13 573, or as follows.

[0186] Since the dehydrogenation reaction takes place with an increasingvolume, the conversion can be increased by reducing the partial pressureof the products. This can be achieved in a simple manner, for example bydehydrogenation at reduced pressure and/or by admixing of substantiallyinert diluent gases, for example steam, which is usually an inert gasfor the dehydrogenation reaction. Another advantage of dilution withsteam is that it generally results in reduced coking of the catalystused since the steam reacts with resulting coke according to theprinciple of gasification of coal. Moreover, steam may be present asdiluent gas in the downstream oxidation step (c). However, steam canalso easily be separated off partly or completely before step (a) (forexample by condensation), which makes it possible to increase theproportion of diluent gas N₂ when the gas mixture obtainable thereby isfurther used in oxidation step (c). Further diluents suitable for thepropane dehydrogenation are, for example, CO, CO₂, nitrogen and noblegases, such as He, Ne and Ar. All diluents stated may be present eitherby themselves or in the form of a very wide range of mixtures. It isadvantageous that said diluents are as a rule also diluents suitable forthe oxidation step (c). In general, diluents which are inert in therespective stage (i.e. which undergo chemical change to an extent ofless than 5, preferably less than 3 and more preferably less than 1, mol%) are preferred. In principle, all dehydrogenation catalysts known inthe prior art are suitable for the propane dehydrogenation. They can bedivided roughly into two groups, i.e. into those which are oxidic innature (for example chromium oxide and/or alumina) and those whichconsist of at least one, as a rule comparatively noble, metal (forexample platinum) deposited on a generally oxidic support.

[0187] Inter alia, all dehydrogenation catalysts which are recommendedin WO 99/46039, U.S. Pat. No. 4,788,371, EP-A-0 705 136, WO 99/29420,U.S. Pat. No. 4,220,091, U.S. Pat. No. 5,430,220, U.S. Pat. No.5,877,369, EP-A-0 117 146, DE-A 199 37 196, DE-A 199 37 105 and DE-A 19937 107 can thus be used. In particular, the catalyst according toexample 1, example 2, example 3 and example 4 of DE-A 199 37 107 can beused.

[0188] These are dehydrogenation catalysts which contain from 10 to99.9% by weight of zirconium dioxide, from 0 to 60% by weight ofalumina, silica and/or titanium dioxide and from 0.1 to 10% by weight ofat least one element of the first or second main group, one element ofthe third subgroup, one element of the eighth subgroup of the PeriodicTable of the Elements, lanthanum and/or tin, with the proviso that thesum of the percentages by weight is 100% by weight.

[0189] In principle, all reactor types and process variants known in theprior art are suitable for carrying out the propane dehydrogenation.Descriptions of such process variants are contained, for example, in allprior art publications mentioned in relation to the dehydrogenationcatalysts.

[0190] A comparatively detailed description of dehydrogenation processessuitable according to the invention is also contained in CatalyticalStudies Division, Oxidative Dehydrogenation and AlternativeDehydrogenation Processes, Study Number 4192 OD, 1993, 430 FergusonDrive, Mountain View, Calif., 94043-5272 U.S.A.

[0191] Typical of partial heterogeneously catalyzed dehydrogenation ofpropane is that it takes place endothermically, i.e. the heat (energy)necessary for establishing the required reaction temperature must besupplied either to the reaction gas beforehand and/or in the course ofthe catalytic dehydrogenation.

[0192] Furthermore, owing to the high reaction temperatures required, itis typical of heterogeneously catalyzed dehydrogenations of propane thatsmall amounts of high-boiling high molecular weight organic compounds,including carbon, are formed and are deposited on the catalyst surfaceand thus deactivate the latter. In order to minimize thisdisadvantageous phenomenon, the propane to be passed over the catalystsurface for the catalytic dehydrogenation at elevated temperatures canbe diluted with steam. Under the resulting conditions, carbon depositedis partly or completely eliminated by the principle of the gasificationof coal.

[0193] Another possibility for eliminating deposited carbon compounds isto pass an oxygen-containing gas through the dehydrogenation catalystfrom time to time at elevated temperatures and thus more or less to burnoff the deposited carbon. Suppression of the formation of carbondeposits is however also possible by adding molecular hydrogen to thepropane to be dehydrogenated catalytically, before it is passed atelevated temperatures over the dehydrogenation catalyst.

[0194] Of course, it is also possible to add steam and molecularhydrogen as a mixture to the propane to be dehydrogenated catalytically.The addition of molecular hydrogen to the catalytic dehydrogenation ofpropane also reduces the undesired formation of allene and acetylene asbyproducts.

[0195] A suitable reactor form for the propane dehydrogenation is thefixed-bed tubular reactor or tube-bundle reactor. This means that thedehydrogenation catalyst is present as a fixed bed in a reaction tube orin a bundle of reaction tubes. The reaction tubes are heated bycombustion of a gas, for example a hydrocarbon, such as methane, in thespace surrounding the reaction tubes. It is advantageous to use thisdirect form of catalyst tube heating only over the initial about 20 to30% of the fixed bed and to heat up the remaining bed length to therequired reaction temperature by the radiant heat liberated in thecourse of the combustion. In this way, an almost isothermal reaction isachievable. Suitable internal diameters of the reaction tubes are fromabout 10 to 15 cm. A typical dehydrogenation tube-bundle reactorcomprises from 300 to 1 000 reaction tubes. The temperature in theinterior of the reaction tube is from 300 to 700° C., preferably from400 to 700° C. Advantageously, the reaction gas is preheated to thereaction temperature before being fed to the tubular reactor.Frequently, the product gas mixture leaves the reaction tube at atemperature of from 50 to 100° C. lower. In the abovementionedprocedure, the use of oxidic dehydrogenation catalysts based on chromiumoxide and/or alumina is expedient.

[0196] Frequently, no diluent gas is present but substantially purepropane is employed as starting reaction gas. The dehydrogenationcatalyst, too, is generally used undiluted.

[0197] On the industrial scale, about three tube-bundle reactors wouldbe operated in parallel. Two of these reactors would as a rule be in thedehydrogenation mode while the catalyst load is regenerated in one ofthe reactors.

[0198] The above procedure is used, for example, in the BASF Lindepropane dehydrogenation process known in the literature.

[0199] Furthermore, it is used in the steam active reforming (STAR)process which was developed by Phillips Petroleum Co. (cf. for exampleU.S. Pat. No. 4,902,849, U.S. Pat. No. 4,996,387 and U.S. Pat. No.5,389,342). The dehydrogenation catalyst used in the STAR process isadvantageously promoter-containing platinum on zinc (magnesium) spinelas a support (cf. for example U.S. Pat. No. 5,073,662). In contrast tothe BASF Linde propane dehydrogenation process, propane to bedehydrogenated in the STAR process is diluted with steam. A molar ratioof steam to propane in the range of from 4 to 6 is typical. Theoperating pressure is frequently from 3 to 8 atom and the reactiontemperature is expediently chosen to be from 480 to 620° C. Typicalcatalyst loadings with the total reaction gas mixture are from 0.5 to 10h⁻¹.

[0200] The propane dehydrogenation can also be designed in the form of amoving bed. For example, the moving catalyst bed can be housed in aradial flow reactor. The catalyst moves therein slowly from top tobottom while the reaction gas mixture flows radially. This procedure isused, for example, in the UOP Oleflex dehydrogenation process. Since thereactors in this process are operated quasi-adiabatically, it isexpedient to operate a plurality of reactors connected in series(typically up to four). This makes it possible to avoid excessivelylarge differences in the temperatures of the reaction gas mixture at thereactor entrance and at the reactor exit (in the case of the adiabaticmode of operation, the reaction gas starting mixture acts as aheat-transfer medium on whose heat content the reaction temperature isdependent) and nevertheless to achieve attractive total conversions.

[0201] When the catalyst bed has left the moving-bed reactor, it isregenerated and then reused. For example, a spherical dehydrogenationcatalyst which substantially comprises platinum on spherical aluminasupports can be used as a dehydrogenation catalyst for this process. Inthe UOP variant, hydrogen is added to the propane to be dehydrogenated,in order to avoid premature catalyst aging. The operating pressure istypically from 2 to 5 atm. The molar hydrogen-to-propane ratio isexpediently from 0.1 to 1. The reaction temperatures are preferably from550 to 650° C. and the time for which the catalyst is in contact withthe reaction gas mixture is chosen to be from about 2 to 6 h⁻¹.

[0202] In the fixed-bed process described, the catalyst geometry maylikewise be spherical or cylindrical (hollow or solid).

[0203] As a further process variant for the propane dehydrogenation,Proceedings De Witt, Petrochem. Review, Houston, Tex., 1992 a, N1,describes the possibility of heterogeneously catalyzed propanedehydrogenation in a fluidized bed, in which the propane is not diluted.

[0204] Expediently, two fluidized beds are operated side by side, one ofwhich is as a rule present in the regeneration state. The activematerial used is chromium oxide on alumina. The operating pressure istypically from 1 to 1.5 atm and the dehydrogenation temperature is as arule from 550 to 600° C. The heat required for the dehydrogenation isintroduced into the reaction system by preheating the dehydrogenationcatalyst to the reaction temperature. The operating pressure is usuallyfrom 1 to 2 atm and the reaction temperature is typically from 550 to600° C. The above dehydrogenation procedure is also known in theliterature as the Snamprogetti-Yarsintez process.

[0205] As an alternative to the procedures described above, theheterogeneously catalyzed propane dehydrogenation in the substantialabsence of oxygen can also be realized according to a process developedby ABB Lummus Crest (cf. Proceedings De Witt, Petrochem. Review,Houston, Tex., 1992, P1). The heterogeneously catalyzed propanedehydrogenation processes in the substantial absence of oxygen whichhave been described to date have in common the fact that they areoperated at propane conversions of >30 mol % (as a rule ≦60 mol %)(based on a single reactor pass). It is advantageous that it issufficient to achieve a propane conversion of from ≧5 to ≦30 or ≦25 mol%. This means that the propane dehydrogenation can also be operated atpropane conversions of from 10 to 20 mol % (the conversions are based ona single reactor pass). This is due, inter alia, to the fact that theremaining amount of unconverted propane is diluted in the downstreamoxidation step (c) with molecular nitrogen, which reduces thepropionaldehdye and/or propionic acid byproduct formation.

[0206] For realizing the abovementioned propane conversions, it isadvantageous to carry out the propane dehydrogenation at an operatingpressure of from 0.3 to 3 atm. It is also advantageous to dilute thepropane to be dehydrogenated with steam. Thus, on the one hand, the heatcapacity of the water makes it possible to compensate some of the effectof the endothermic nature of the dehydrogenation and, on the other hand,the dilution with steam reduces the partial pressure of startingmaterials and products, which has an advantageous effect on theequilibrium position of the dehydrogenation. Furthermore, as statedabove, the presence of steam has an advantageous effect on thetime-on-stream of the dehydrogenation catalyst. If required, molecularhydrogen may also be added as a further component. The molar ratio ofmolecular hydrogen to propane is as a rule <5. With a comparatively lowpropane conversion, the molar ratio of steam to propane can accordinglybe from >0 to 30, expediently from 0.1 to 2, advantageously from 0.5to 1. The fact that only a comparatively small amount of heat isconsumed in a single reactor pass of the reaction gas and comparativelylow reaction temperatures are sufficient for achieving the conversion ina single reactor pass proves to be advantageous for a procedure having alow propane conversion.

[0207] It may therefore be expedient to carry out the propanedehydrogenation (quasi) adiabatically with a comparatively low propaneconversion, i.e. the reaction gas starting mixture is heated as a ruleto a temperature of from 500 to 700° C. (for example by direct firing ofthe surrounding wall) or from 550 to 650° C. Usually, a single adiabaticpass through a catalyst bed is then sufficient to achieve the desiredconversion, the reaction gas mixture being cooled by from about 30 to200° C. (depending on conversion). The presence of steam as aheat-transfer medium is advantageous even from the point of view of anadiabatic procedure. The lower reaction temperature permits longertimes-on-stream of the catalyst bed used.

[0208] In principle, the propane dehydrogenation with comparatively lowpropane conversion, whether adiabatically or isothermally operated, canbe carried out both in a fixed-bed reactor and in a moving-bed orfluidized-bed reactor.

[0209] It is noteworthy that a single shaft furnace reactor in the formof a fixed-bed reactor through which the reaction gas mixture flowsaxially and/or radially is sufficient for realizing saiddehydrogenation, in particular in adiabatic operation.

[0210] In the simplest case, this is a single closed reaction volume,for example a container, whose internal diameter is from 0.1 to 10 m,possibly also from 0.5 to 5 m, and in which the catalyst bed isinstalled on a support apparatus (for example a grille). The reactionvolume which is loaded with catalyst and is heat-insulated in adiabaticoperation is flowed through axially by the hot, propane-containingreaction gas. The catalyst geometry may be spherical, annular orstrand-like. Since in this case the reaction volume is to be realized bya very economical apparatus, all catalyst geometries which have aparticularly low pressure drop are preferable. These are in particularcatalyst geometries which lead to a large cavity volume or are built upin a structured manner, for example honeycombs. In order to realizeradial flow of the propane-containing reaction gas, the reactor mayconsist, for example, of two cylindrical grilles present in a casing andmounted concentrically one inside the other, and the catalyst bed may bearranged in their annular gap. In the adiabatic case, the metal casingin turn would be thermally insulated.

[0211] The catalysts disclosed in DE-A 199 37 107, especially all thosedisclosed by way of example, are particularly suitable as a catalystload for the propane dehydrogenation with comparatively low propaneconversion in a single pass.

[0212] After a relatively long operating time, the abovementionedcatalysts can be regenerated, for example, in a simple manner by passingair diluted with nitrogen over the catalyst bed at from 300 to 600° C.,frequently from 400 to 500° C., initially in the first regenerationstages. The catalyst loading with regeneration gas may be, for example,from 50 to 10 000 h⁻¹ and the oxygen content of the regeneration gas maybe from 0.5 to 20% by volume.

[0213] In further downstream regeneration stages, air can be used asregeneration gas under otherwise identical regeneration conditions. Itis expedient in application technology to flush the catalyst with inertgas (for example N₂) before its regeneration.

[0214] Thereafter, it is generally advisable to effect regeneration withpure molecular hydrogen or with molecular hydrogen diluted with inertgas (the hydrogen content should be ≧1% by volume) under an otherwiseidentical range of conditions.

[0215] The propane dehydrogenation with comparatively low propaneconversion (≦30 mol %) can be operated in all cases at the same catalystloadings (relating both to the reaction gas as a whole and the propanecontained therein) as the variants with high propane conversion (>30 mol%). This loading with reaction gas may be, for example, from 100 to 10000 h⁻¹, frequently from 100 to 3 000 h⁻¹, i.e. often from about 100 to2 000 h⁻¹.

[0216] The propane dehydrogenation with comparatively low propaneconversion can be realized in a particularly elegant manner in a trayreactor.

[0217] This contains, spatially in succession, more than one catalystbed catalyzing the dehydrogenation. The number of catalyst beds may befrom 1 to 20, expediently from 2 to 8, but also from 3 to 6. Thecatalyst beds are preferably arranged radially or axially one behind theother. In terms of application technology, it is expedient to use thefixed catalyst bed type in such a tray reactor.

[0218] In the simplest case, the fixed catalyst beds in a shaft furnacereactor are arranged axially or in the annular gaps of cylindricalgrilles installed concentrically one inside the other. However, it isalso possible to arrange the annular gaps in segments and, after radialpassage through a segment, to pass the gas into the next segment aboveor below.

[0219] Expediently, the reaction gas mixture is subjected tointermediate heating in the tray reactor on its way from one catalystbed to the next catalyst bed, for example by passing it over heatexchanger ribs heated with hot gases or by passing it through pipesheated with hot combustion gases.

[0220] If the tray reactor is otherwise operated adiabatically, it issufficient for the desired propane conversions (≦30 mol %), particularlywith the use of the catalysts described in DE-A 199 37 107, inparticular the exemplary embodiments, to preheat the reaction gasmixture to a temperature from 450 to 550° C. before passing it into thedehydrogenation reactor and to keep it within this temperature rangeinside the tray reactor. This means that the total propanedehydrogenation is thus to be realized at extremely low temperatures,which proves to be particularly advantageous for the time-on-stream ofthe fixed catalyst beds between two regenerations.

[0221] It is even more elegant to carry out the intermediate heatingdescribed above by a direct method (autothermal procedure). For thispurpose, a limited amount of molecular oxygen is added to the reactiongas mixture, before it flows through the first catalyst bed and/orbetween the downstream catalyst beds. Depending on the dehydrogenationcatalyst used, limited combustion of the hydrocarbons contained in thereaction gas mixture, any coke deposited on the catalyst surface orcoke-like compounds and/or hydrogen formed in the course of the propanedehydrogenation and/or added to the reaction gas mixture is thuseffected (it may also be expedient in terms of application technology tointroduce into the tray reactor catalyst beds which are loaded withcatalyst which specifically (selectively) catalyzes the combustion ofhydrogen (and/or of hydrocarbon) (suitable catalysts of this type are,for example, those of U.S. Pat. No. 4,788,371, U.S. Pat. No. 4,886,928,U.S. Pat. No. 5,430,209, U.S. Pat. No. 5,530,171, U.S. Pat. No.5,527,979 and U.S. Pat. No. 5,563,314; for example, such catalyst bedscan be housed in the tray reactor so as to alternate with the bedscontaining the dehydrogenation catalyst)). The heat of reaction evolvedthus permits virtually isothermal operation of the heterogeneouslycatalyzed propane dehydrogenation in a quasiautothermal manner. As thechosen residence time of the reaction gas in the catalyst bed increases,a propane dehydrogenation with decreasing or substantially constanttemperature is possible, which permits particularly long times-on-streambetween two regenerations.

[0222] As a rule, an oxygen feed as described above should be effectedso that the oxygen content of the reaction gas mixture is from 0.5 to10% by volume, based on the amount of propane and propene containedtherein. Suitable oxygen sources are both pure molecular oxygen andoxygen diluted with inert gas, for example CO, CO₂, N₂ or noble gases,in particular air. The resulting combustion gases generally have anadditional diluting effect and thus promote the heterogeneouslycatalyzed propane dehydrogenation.

[0223] The isothermal nature of the heterogeneously catalyzed propanedehydrogenation can be further improved by mounting closed internals(for example annular ones), advantageously but not necessarilyevacuated, in the spaces between the catalyst beds in the tray reactorbefore they are introduced. Such internals may also be placed in therespective catalyst bed. These internals contain suitable solids orliquids which evaporate or melt above a specific temperature and thusconsume heat and condense and thereby liberate heat where thetemperature falls below this temperature.

[0224] One possibility of heating the reaction gas mixture to therequired reaction temperature in the propane dehydrogenation is also tocombust a part of the propane and/or H₂ contained therein by means ofmolecular oxygen (for example, over suitable combustion catalysts havinga specific action, for example by simply passing over and/or passingthrough) and to effect heating to the desired reaction temperature bymeans of the heat of combustion thus liberated. The resulting combustionproducts, such as CO₂ and H₂O, and any N₂ accompanying the molecularoxygen required for the combustion advantageously form inert diluentgases.

[0225] According to the invention, it is also possible for propaneunconverted and optionally propene after carrying out step (c) andseparating off the desired product (acrolein and/or acrylic acid) to besubjected to propane dehydrogenation, which can be carried out asdescribed above, and for the product gas mixture obtained after thepropane dehydrogenation to be subjected again to step (a).

[0226] Where a propane dehydrogenation is carried out, propane is apossible diluent gas in step (c).

[0227] It is also possible to add, in particular when a propanedehydrogenation is carried out, to gas B supplied to step (c) still purepropane and/or propene.

[0228] If step (c) is carried out as a conversion of propene to acrylicacid, then the exit gas from the working-up also contains oxidizablesecondary components, for example carbon monoxide, formic acid,formaldehyde, acetic acid and small amounts of acrylic acid in additionto the components not converted in the oxidation, i.e. propane, nitrogenand residual oxygen. In a particularly preferred embodiment, thesesecondary components are catalytically oxidized before a propanedehydrogenation with the residual oxygen and, if required, withadditional molecular oxygen, in order to heat up the gas before thedehydrogenation. This oxidation could be carried out in a postcombustioncatalyst, such as a Pd catalyst on an alumina support, for exampleRO-20/13 or RO-20/25 (both from BASF).

[0229] Preferred embodiments of the invention are shown in FIGS. 1 to 7described below, which illustrate the invention without restricting it.

[0230] FIGS. 1 to 5 show schematic diagrams for carrying out preferredprocesses, in which, for simplification, not all feed and dischargestreams are shown. FIG. 1 shows an absorption and desorption stage 1, anoxidation stage 2, which is in the form of an oxidation of propene toacrolein and/or acrylic acid, and a working-up stage 3. In FIG. 1,propane and propene, if required with residual amounts of nitrogen, areabsorbed into a suitable absorbent in stage 1 from a mixture whichcontains propane, propene, hydrogen and oxides of carbon (carbonmonoxide and carbon dioxide) and possibly nitrogen and furtherhydrocarbons, and are desorbed from said absorbent by stripping withair. In this way, hydrogen, the oxides of carbon, further hydrocarbonsand nitrogen are removed. The stream containing propene and possiblepropane is then fed to the oxidation stage 2, in which propene isoxidized to acrolein and/or acrylic acid. After the oxidation 2, theproduct obtained is fed to the working-up 3. There, the desired productsacrolein and/or acrylic acid are isolated. The remaining unconvertedpropene and propane and oxides of carbon and any residues of nitrogenand oxygen are once again fed to the absorption and desorption stage 1.

[0231] In the further figures, identical reference numerals denote thesame as in FIG. 1.

[0232] In contrast to FIG. 1, in FIG. 2 a propane dehydrogenation 4 isinstalled upstream and can be carried out with or without a supply ofoxygen. The gas mixture obtained in the propane dehydrogenation andcontaining hydrogen, oxides of carbon and possibly residues of nitrogenand hydrocarbons in addition to the propane and the propene is fed tothe absorption and desorption stage 1.

[0233] In contrast to FIG. 1, in FIG. 3 a propane oxidation stage 22, inwhich propane is oxidized to acrolein and/or acrylic acid, is presentinstead of propene oxidation 2.

[0234] In FIG. 4, a propane dehydrogenation stage 4 with or without asupply of oxygen is carried out after the working-up stage 3 and the gasmixture obtained in this stage is recycled to the absorption anddesorption stage 1.

[0235]FIG. 5 shows a further preferred embodiment of the process, inwhich a propane dehydrogenation 5 with an oxygen supply is installeddownstream of the absorption and desorption stage 1.

[0236]FIGS. 6 and 7 show further preferred processes. The process ofFIG. 6 follows the process diagram of FIG. 4. In FIG. 4, three reactorsare present in the propane dehydrogenation stage 4, in the first ofwhich a carbon monoxide postcombustion (CO-PC) takes place before thepropane dehydrogenation (PDH), while in the two downstream reactors ahydrogen postcombustion (H2-PC) takes place before the propanedehydrogenation (PDH). These postcombustions serve for supplying energyfor the propane dehydrogenation. The number of reactors in the propanedehydrogenation is not limited to three reactors. In the propanedehydrogenation 4, propane is fed in via line (30). Air can be fed invia line (6). The gas mixture obtained after the propane dehydrogenationis fed via a heat exchanger W and a compressor V in stage 1 to anabsorption column K1 and a desorption column K2. After the desorption inK2, the absorbent is recycled to absorption column K1. Unabsorbed gasesare removed from the process as waste gas (33), if necessary via anincineration plant E. The stream containing separated off propane and/orpropene is fed to the oxidation stage 2, which is shown here with fouroxidation reactors. However, the number of oxidation reactors is notlimited to this number. The desired product acrolein and/or acrylic acidis then worked up in stage 3 and is taken off via line (31). Unconvertedpropane and/or propene is recycled as recycle gas via line (32),together with the other gaseous components not separated off here in theabsorption, to the propane dehydrogenation 4.

[0237] In FIG. 7, recycle gas (1) from the working-up stage 3, which gasis obtained at from 10 to 90° C. and from 0.8 to 5 bar and can befurther compressed to pressures of from 2 to 10 bar, for example withthe aid of a compressor V0, is heated, in a heat exchanger W1countercurrently to the reaction gas (2) from the propanedehydrogenation (PDH) 4, to temperatures of from 100 to 650° C. In thecase of FIG. 7, the stated pressure in bar relates here and below toabsolute pressure.

[0238] Suitable compressors are all suitable embodiments which are knownto a person skilled in the art and are mentioned in more detail below.

[0239] The recycle gas stream (1) contains from about 40 to 80% byvolume of N₂, from about 1 to 5% by volume of CO₂, from 0.1 to 2% byvolume of CO, from 2 to 5% by volume of O-₂, from 0.5 to 25% by volumeof H₂O, further oxidation byproducts and from about 5 to 40% by volumeof unconverted propane and from about 0.1 to 3% by volume of unconvertedpropene. Before or after the heating-up, fresh propane (3) andpreferably water or steam (4) are mixed with the gas before it is passedinto the PDH 4. Suitable fresh propane is any availablepropane-containing gas or liquid. However, propane sources such asindustrial propane (>90%, in particular >95%, particularlypreferably >98%, propane content with a small C₄ ⁺ fraction) or purepropane (>99% propane content) are advantageous. The molar ratio ofwater or steam to propane in the gas stream is from 0.05 to 5,preferably from 0.1 to 2. It may be advantageous to mix with this gasstream additionally H₂ (5), air (6) or an O₂-containing gas and furthercomponents capable of exothermic conversion in the PDH, for example COor CO/H₂ mixtures, such as synthesis gas. The purity of these gases isnot subject to any restriction. The exothermic nature of the oxidationof the combustible components in the PDH serves for covering theendothermic nature of the PDH reaction, so that less additional heat,and in the most advantageous case no additional heat has to be suppliedfrom outside for covering the enthalpy of reaction for the PDH.

[0240] The PDH is operated at from 0.3 to 10, preferably from 1 to 5,bar and from 350 to 700° C., preferably from 400 to 600° C. Possiblereactors for the PDH are all embodiments known to a person skilled inthe art, for example axial-flow apparatuses, such as tray reactors, andalso apparatuses having a plurality of catalyst beds which are arrangedin the form of a hollow cylinder and having radial flow, or a pluralityof individual apparatuses, for example column-type, cake-type orspherical apparatuses. The number of reactors in the PDH is not limitedto 3. Preferably, a plurality of individual apparatuses is used sincethe intermediate feeding of further gases is possible in a simple mannerthereby and moreover individual catalyst beds can be treated in aparticular manner, for example regenerated, separately from the othersduring operation. For this purpose, for example, the reactor containingthe catalyst bed to be regenerated is isolated from the main gas streamby suitable shut-off elements, for example slide valves, valves or flapswhich are present in the connecting lines between the reactors, and thegases required for regeneration, for example N₂, H₂, lean air or air orO₂-rich gases, are then passed over the catalyst bed and deposits areremoved from the catalyst. The remaining reactors, a total number ofwhich may be from 1 to 20, preferably from 2 to 5, are still fed withthe main gas stream and produce mainly the desired product propene.

[0241] In the PHD reactors, the catalyst layers may rest on grilles,beds of inert material or similar support apparatuses known to a personskilled in the art. The form of the catalyst is not subject to anyrestrictions. Forms such as chips, spheres, extrudates, rings, cylindersor structured packings and monoliths may be used. Those geometries whichprovide a small pressure drop are advantageous.

[0242] For the distribution of the gas fed in over the catalyst bed, gasdistributors known to a person skilled in the art, for example sievetrays, ring distributors or manifolds, and irregular beds or structuredpackings, for example static mixers, may be used.

[0243] A plurality of catalyst layers having different functions may bearranged in the PDH reactors. If a plurality of catalyst layers areused, it is advantageous to arrange, before the PDH catalyst layer, oneor more catalyst layers over which preferably, for example, H₂, COand/or a further oxidizable component which is not propene or propanecan be oxidized (CO—PC or H₂—PC). However, it is also possible todispense with additional catalyst layers upstream of the PDH catalystlayer if the PDH catalyst performs this function or propane lossesthrough oxidation with propane are economically acceptable.

[0244] The PDH catalysts can be operated with from 100 to 20 000,preferably from 500 to 10 000, more preferably from 1 000 to 10 000,l(S.T.P.) of propane per liter of catalyst bed per hour. The gas spacevelocity for catalysts which oxidize, for example, predominantly CO orH₂ and to a lesser extent propane or propene is usually from 5 000 to 30000 l(S.T.P.) of gas per liter of catalyst bed per hour.

[0245] The propane conversion in the PDH is from 10 to 60%, preferablyfrom 20 to 50%, at propane selectivities of from 80 to 99.5%, frequentlyfrom 88 to 96%. The conversion of the feed gases CO, H₂ or othercombustion gases is advantageously complete. The conversion of H₂ formedduring the PDH is from 1 to 99%, often from 10 to 80%, frequently from30 to 70%, depending on the propane conversion.

[0246] The reaction gas (2) from the PDH contains from about 20 to 60%by volume of N₂, from about 1 to 5% by volume of CO₂, from 0.5 to 45% byvolume of H₂O, from about 5 to 40% by volume of propane, from about 1 to20% by volume of propene, from about 1 to 20% by volume of H₂ andfurther byproducts, for example ethane, ethene, methane and C₄ ⁺.

[0247] The reaction gas (2) from the PDH is obtained at from 400 to 650°C., more advantageously from 450 to 600° C., and from 0.3 to 10, moreadvantageously from 1 to 5, bar. It is cooled, countercurrently to therecycle gas (1), to temperatures which are at least 5° C., better atleast 50° C., and preferably at least 100° C., above the inlettemperature of the recycle gas (1). The gas stream (3) is then furthercooled in one or more stages to about 10 to 60° C., depending on thetemperature on emergence from the countercurrent heat exchanger W1.

[0248] In the multistage cooling, the cooling in W2 can be effected bysteam generation or by air cooling and the cooling in W3 by air, wateror brine cooling, depending on the temperature level. Depending on thepressure, temperature and H₂O content in the gas streams (3) to (5),water condenses and is separated from the gas stream (5) in theseparator A1. Suitable gas separators are all embodiments known to aperson skilled in the art and suitable for this purpose.

[0249] The cooled and if necessary partly dewatered gas stream (6) isthen compressed to pressures from the pressure on emergence from theseparator A1 to 50 bar. The compression can be effected either in onestage or in a plurality of stages with or without intermediate cooling.Suitable compressors V1 are all embodiments known to a person skilled inthe art and suitable for this purpose, for example reciprocating androtary compressors, screw-type compressors, diaphragm-type compressors,rotary multi-vane compressors, turbo compressors, centrifugalcompressors and rotary piston blowers and centrifugal blowers; however,turbo compressors or centrifugal compressors are preferably used.Criteria for choosing the compressors are both the pressure increase andthe amount of gas stream to be compressed. In the multistage compressionwith intermediate cooling, water and possibly further condensablecomponents condense during the intermediate cooling and can be separatedfrom the gas stream during or after the intermediate cooling asdescribed above, before the gas stream is fed to the next compressorstage. The gas stream (7) compressed to the final pressure can be cooledagain as described above in one or more stages, it being possible onceagain for water and any other condensable substances to be separatedfrom the gas streams (7) to (9).

[0250] The compressor V1 may be operated both by means of electricmotors and by means of steam or gas turbines. The choice depends on theinfrastructure conditions. Frequently, driving by means of steam turbineproves most economical.

[0251] The sum of the condensed streams, for example (11)—after apressure increase—and (12), is recirculated to the PDH, to the extentthat is required for covering the H₂O-to-propane ratio before entry intothe PDH, and the remainder is discharged and if necessary incinerated.The condensate stream (13) can be vaporized before recirculation ordischarge or can be subjected to a further treatment, for example apurification, before recirculation.

[0252] The gas stream (10) is then fed to the absorption column K1, inwhich propane and/or propene are separated from the gas stream. Here,the gas stream (10) is brought into contact with an absorbent, whichtakes up the C₃ fraction and may take up further components. Suitableabsorbents are all substances known to a person skilled in the art, theabsorbents described above preferably being used. The gas stream (10) ispreferably fed countercurrently to the absorbent in a plurality ofstages. The absorption can be effected at from 10 to 150° C., better atfrom 20 to 80° C., preferably at from 30 to 60° C., and at from 1 to 50,better at from 3 to 30, preferably at from 5 to 20, bar.

[0253] Suitable absorbers K1 are all embodiments known to a personskilled in the art, as described, for example, in ThermischeTrennverfahren; Klaus Sattel, VCH, 1988 (ISBN 3527-28636-5). Columnshaving internals are preferable. Suitable internals are likewise allembodiments known to a person skilled in the art, for example sievetrays, dual-flow trays, bubble trays, tunnel trays, lattice trays, valvetrays or irregular beds, for example comprising rings (for example fromRaschig), Pall rings, Intalox saddles, Berl saddles, super saddles,toroidal saddles, Interpack packing or wire mesh rings and structuredpackings (for example Sulzer-Kerapak or Sulzer packing BX or CY, or, forexample, packings from Montz and packings from other manufacturers).Ralu-Pak 250.YC from Raschig is particularly suitable. Internals whichpermit high liquid loading or irrigation density, for exampleunstructured beds or structured packings, are preferable. The possibleirrigation density should be greater than 50, preferably greater than80, m³ of liquid per m² of free cross-sectional area per hour. Theinternals may be either metallic, ceramic or of plastic or may consistof a composition comprising a plurality of materials. What is importantin the choice of the material for the beds and packings is that theabsorbent thoroughly wets these internals.

[0254] The ratio of the streams between the absorptive (24) fed to theabsorption and gas stream (10) follows the thermodynamic requirementsand depends on the number of theoretical plates, the temperature, thepressure, the absorption properties of the absorbent and the requireddegree of separation. Ratios of from 1:1 to 50:1, in particular from 1:1to 30:1, preferably from 1:1 to 20:1, in kg/kg, with from 1 to 40, inparticular from 2 to 30, preferably from 5 to 20, theoretical plates,are usual. The definition of a theoretical plate appears in thetechnical literature, for example “Thermische Trennverfahren”, KlausSattel, VCH, 1988, (ISBN 3-527-28636-5).

[0255] The gas stream (14) in which the concentration of propane and/orpropene has been reduced can be fed to a quench stage in order, ifrequired, to reduce absorbent losses. The mode of operation of a quenchis explained in more detail below in the description of the desorptionstage.

[0256] After leaving any quench stage, the gas stream (14) can be letdown. Letting down can be effected either in one stage or in a pluralityof stages by throttling without energy recovery, or in one or morestages in a gas turbine T1 with recovery of mechanical energy. In thecase of the recovery of mechanical energy, it may be necessary to heatup the gas stream (14) before it is passed into the turbine. The gasstream can be heated up both directly by catalytic and noncatalyticoxidation of combustible and oxidizing components contained in the gasstream or fed in from outside, and by indirect heat supply with the aidof steam or external firing. The mechanical energy obtained duringlet-down can be used directly as a concomitant or main means for drivingone of the compressors, preferably V1, or for generating electric power.

[0257] After the let-down, the waste gas stream (15) obtained can,depending on its purity, be fed to a catalytic or noncatalytic waste gasincineration or discharged directly into the atmosphere.

[0258] The absorbent stream (16) laden predominantly with propane (from2 to 30% by volume) and/or propene (from 2 to 30% by volume) andpossibly further components (for example CO₂, C₂ ⁻, C₄ ⁺, H₂O), is letdown if necessary and then fed to the desorption column K2. The let-downcan be effected both without recovery of the mechanical energy in one ormore stages and with recovery of mechanical energy (for example in aturbine or a centrifugal pump operating in reverse). Moreover, it may beuseful to heat up the stream (16) prior to desorption. This heating-upis preferably effected by means of countercurrent heat exchange with thestream (17) in W6. In addition, it may be useful to heat up stream (16)over and above this.

[0259] The desorption in K2 of propane and/or propene can be carried outby distillation, by simple flash or by stripping. The desorption issupported by reducing the pressure to 0.1 to 10, in particular to 1 to5, preferably to 1.5 to 3, bar.

[0260] If the desorption is effected by distillation, the separationstep can be carried out on the basis of all knowledge known to a personskilled in the art. A particularly simple embodiment of the desorptionis the one-stage flash or flash evaporation of the laden solvent in anapparatus suitable for this purpose. It may be expedient to heat thestream (16) to 20 to 300° C., in particular to 40 to 200° C., preferablyto 50 to 150° C., prior to flashing. The apparatus should be designed sothat both the thermodynamic separation between propane or propene andthe solvent and the fluid dynamic separation between gas and liquid takeplace readily. The apparatus may have, for example, a cylindrical orspherical shape, as well as other designs known to a person skilled inthe art. If the apparatus is of a cylindrical shape, the cylinder may beeither upright or horizontal. Viewed vertically, the feed to the flashapparatus is as a rule between the gas discharge and the liquiddischarge. In the simplest case, the apparatus has no additionalinternals. For better thermodynamic separation, the internals such asthose known to a person skilled in the art for distillation, absorptionand stripping can be installed in the apparatus, in particular thosedescribed in the text above for the absorption. For better fluid dynamicseparation, internals such as those known to a person skilled in the artfor gas/liquid separation, for example knitted fabrics, deflector platesor the like, may additionally be integrated in the flash apparatus.Moreover, the flash apparatus may contain apparatuses which permit theintroduction of heat, for example heated pipe coils or heated walls.

[0261] If, as in the present case, air or a similar stripping medium(for example steam, N₂, fresh propane or a further gas required in theprocess) is available, it is expediently used for supporting the flashprocess.

[0262] A special embodiment for this purpose is the multistage strippingof the volatile components propane and/or propene with the starting gasstream (25) for the oxidation (of course, as in the one-stage case, alladditionally absorbed components from the stream (10) and any substancesformed are also stripped according to their volatility). In the simplestcase, the starting gas stream (25) is the air required for oxidizing thepropene to acrolein or acrylic acid. The compression of the air or ofthe starting gas stream can be effected both before and after thedesorption. However, the starting gas stream (25) may also containrecycled gas from the acrylic acid process and steam, fresh propane orfurther blanketing gaseous component in addition to the air. It isparticularly advantageous if the starting gas stream (25) is fedcountercurrently or crosswise to the liquid absorbent during thedesorption. In the case of countercurrent flow, the desorption apparatusor desorber may be designed in the same way as the absorption columndescribed in the text above.

[0263] The cross-flow may be expedient if the explosion range is passedthrough during the desorption. This is the case when the starting gasstream (25) is a lean gas mixture with respect to the combustiontendency and the gas stream (18) laden with propane and/or propene is arich gas mixture with respect to the combustion inclination after thedesorption. The gas mixture is defined as being lean in this contextwhen the content of combustible substances is too low to be ignitableand a gas mixture is defined as rich in this context when the content ofcombustible substances is too high to be ignitable.

[0264] In the case of cross-flow, the total starting gas stream is notintroduced into the bottom but is divided into part-streams andintroduced at a plurality of suitable points along the desorptioncolumn, this being done in such a way that an ignitable gas mixture isnot present at any point in the desorption apparatus. The desorptioncolumn may be arranged vertically or horizontally.

[0265] A further possibility for overcoming the explosion problem in thedesorption of combustible components with O₂-containing gas streams, isto mix the starting gas stream, prior to entry into the desorptioncolumn, with a substance (for example propane, propene, methane, ethane,butane, H₂O, etc.) in such a way that the starting gas mixture is itselfrich prior to entry into the desorption column. However, it is alsopossible to split the starting gas stream and to pass a propane- orpropene-free starting gas into the bottom of the desorption column inorder to achieve very good depletion of propane and/or propene in theabsorptive (17) and to pass a starting gas which, for example, isenriched with propane and/or propene into that region of the desorptioncolumn in which an ignitable gas mixture can form.

[0266] After any countercurrent heat exchange (W6) and a pressureincrease with the aid of a pump (P1), the absorbent stream (17) depletedin propane and/or propene can be further cooled in one or more stages(for example in W7) and fed via line (24) back to the absorber K1.

[0267] In general, the multistage desorption may take place at allpressures and temperatures.

[0268] However, pressures which are lower, and temperatures which arehigher, than those in the absorption are advantageous. In the presentcase, pressures of from 1 to 5, in particular from 2 to 3, bar andtemperatures of from 20 to 200° C., in particular from 30 to 100° C.,particularly preferably from 35 to 70° C., are desirable.

[0269] The ratio of absorbent stream (17) to starting gas stream (25)follows the thermodynamic requirements and depends on the number oftheoretical plates, the temperature, the pressure and the desorptionproperties of the absorbent and the required degree of separation.Ratios of from 1:1 to 50:1, in particular from 5:1 to 40:1, preferablyfrom 10:1 to 30:1, in kg/kg with from 1 to 20, in particular from 2 to15, preferably from 3 to 10, theoretical plates are usual.

[0270] In general, the starting gas stream laden with propane and/orpropene can be fed without further treatment to the oxidation stages 2.However, it may be expedient to feed the starting gas stream, prior tothe oxidation, to a further process stage in order, for example, toreduce the losses of concomitantly stripped absorbent. The separation ofthe absorbent from the laden starting gas stream for the oxidation canbe carried out by all process steps known to a person skilled in theart. One possible embodiment is the quenching of the laden starting gasstream with water. In this case, the absorbent is washed out of theladen starting gas stream with water. This washing or quenching can becarried out at the top of the desorption column over a liquid collectingtray or in a separate apparatus. Internals such as those known to aperson skilled in the art for distillation, absorption and desorptionand as described in the text above for the absorption can be installedin the quench apparatus for supporting the separation effect. The sameapplies to the quench apparatus where it is designed as a separateapparatus.

[0271] After the water has washed the absorbent out of the starting gasstream laden with propane and/or propene, the water/absorbent mixture(19) can be fed to a phase separation D1, and the treated starting gasstream (18), after possible preheating, can be fed to the propeneoxidation stage 2.

[0272] The phase separation can be carried out in all embodiments knownto a person skilled in the art, as also used, for example, inliquid/liquid extraction. In the simplest case, these are horizontal orvertical elongated apparatuses with or without internals, in which theorganic absorbent phase separates from the quench water. Thediameter-to-length ratio here may be from 1:1 to 1:100, in particularfrom 1:1 to 1:10, preferably from 1:1 to 1:13. The apparatus may beflooded or may be operated using a gas cushion. For better isolation ofthe organic absorbent phase, the apparatus can be equipped with a domefrom which the organic phase can be taken off. For supporting the phaseseparation, all internals known to a person skilled in the art for thispurpose, for example knitted fabrics, wound cartridges or deflectorplates, may be installed. Of course, rotating phase separators, forexample centrifuges, may also be used.

[0273] After the phase separation, the absorptive (20) separated off canbe recycled to the desorption. The quench water can, if required, becooled or heated in a heat exchanger (W9) before reentering the quenchapparatus. Advantageously, large amounts of water are circulated withthe aid of a pump (P2). Suitable irrigation densities in the quenchapparatus are greater than 30, in particular greater than 50, preferablygreater than 80, but less than 1 000, in particular 500, preferably lessthan 300, m³ of water per m² of free cross-sectional area of the quenchapparatus per hour.

[0274] The water losses during quenching can be covered by condensate(21) as well as by dilute acid solution (22) from the acrylic acidpreparation process. In order to avoid increasing concentrations, a partof the circulation quench water can be removed as a purge stream (23)and fed to the incineration plant or to another treatment for disposal(for example in a wastewater treatment plant).

[0275] If the gas stream (18) for the propene oxidation 2 has atemperature of <90° C., in particular <70° C. at pressures of from 1 to3 bar, it may be expedient also to add water to this stream. This can beeffected by admixing steam or by saturating the stream (18) in a watersaturator in a manner known to a person skilled in the art. The gasstream treated in this manner has a composition of from 30 to 70% byvolume of N₂, from 5 to 20% by volume of O₂, from 2 to 15% by volume ofpropene, from 2 to 40% by volume of propane and from 0.5 to 25% byvolume of H₂O and contains further components, for example CO₂, methane,ethane, ethene and C₄ ⁺. It can be fed to the oxidation 2, which can becarried out as described above or as disclosed in the patent literature.The propene or acrolein oxidation can be carried out in salt bathreactors, for example from Deggendorfer-Werft according to the prior artor in other reactor types. An air feed or steam feed may or may not onceagain take place between the first oxidation stage to acrolein and thesecond oxidation stage to acrylic acid. The higher C₃ content in the gas(18) with the oxidation 2 in any case requires removal of the heat ofreaction from the reaction space. Propene loadings of from 5 to 350, inparticular from 90 to 300, preferably from 100 to 250, l(S.T.P.) ofpropene per liter of catalyst bed per hour are suitable.

[0276] The separation of the acrylic acid from the reaction gas (26) ofthe oxidation 2 in stage 3 can be carried out as described above using,for example, a high-boiling solvent, such as a mixture of diphyl anddimethyl phthalate, or by absorption in water and by fractionalcondensation.

[0277] The purification of the acrylic acid can be effected by strippingand distillation or by azeotropic distillation or by crystallization.

[0278] The process described in FIG. 7 is suitable both for retrofittingof all existing plants for the production of acrolein and/or acrylicacid and in combination with new acrylic acid plants.

[0279] Surprisingly, it was found that, in spite of the usually expectedresidues of absorbent in the gas B, no problems occurred with theoxidation or with the oxidation catalyst. Moreover, no problems wereobserved with any oxidation products which may form from the absorbentduring the oxidation. Where problems occur with residues of absorbent,which as a rule is not the case when hydrocarbons having a high boilingpoint, in particular paraffins, are used as absorbent, said absorbentcan be removed, for example by a water quench or by adsorption.

[0280] It was therefore surprising that absorption can be used in thenovel process. In contrast to the adsorption used in the Japanesepublication JP-A-10 36311, the absorption used here for propane and/orpropene is substantially easier and more economical to handle.

[0281] Furthermore, the present invention has the advantage thatexisting plants for the preparation of acrolein and/or acrylic acidwhich use propene as a starting material can be converted in anadvantageous manner to the more economical propane as a startingmaterial.

[0282] The example which follows and which describes the preferredembodiment of the novel process illustrates the invention.

EXAMPLE

[0283] Acrylic acid is prepared by a process as shown in FIG. 7. Thereference numerals used below therefore relate to FIG. 7.

[0284] 2 090 l(S.T.P.)/h of the recycle gas (1) from the working-upstage 3, which is obtained at a temperature of 30° C. and a pressure of1.2 bar, are compressed to 2.0 bar with the aid of a compressor V0 andheated to 450° C. in a heat exchanger W1 countercurrently to thereaction gas (2) from the propane dehydrogenation (PDH). The statedpressure in bar relates here and below in this example to the absolutepressure.

[0285] The recycle gas stream (1) contains 60.3% by volume of N₂, 1.2%by volume of CO₂, 0.5% by volume of CO, 3.4% by volume of O₂, 1.9% byvolume of H₂O, 32.2% by volume of propane and 0.4% by volume of propeneand further oxidation byproducts. Before the heating-up, 170 l(S.T.P.)/hof fresh propane (3) and steam (4) are mixed with the recycle gas streambefore it is passed into the PDH. The fresh propane used is industrialpropane (>98% propane content with 100 ppm by weight of C₄ ⁺fraction).The molar ratio of steam to propane in the gas stream is 0.5.

[0286] The gas mixture is fed to the 1st reactor of 4 reactors. Theinternal diameter of the reactors is 50 mm. The reactors are designed insuch a way that they can be operated autothermally. Each reactorcontains a 110 mm high catalyst bed comprising extrudates (d=3 mm, 1=5mm).

[0287] In the 4 reactors, propane undergoes 20% conversion at a propeneselectivity of 92%.

[0288] The reaction gas (2) from the PDH contains 44.9% by volume of N₂,2.7% by volume of CO₂, 16.9% by volume of H₂O, 24.0% by volume ofpropane, 5.8% by volume of propene, 5.5% by volume of H₂ and smallamounts of further byproducts, for example ethane, ethene, methane andC₄ ⁺.

[0289] The reaction gas (2) from the PDH is obtained at 520° C. and 1.5bar and is cooled to 30° C. On average, about 350 g of water (11) perhour condense.

[0290] The cooled gas stream (6) is then compressed in one stage in apiston compressor to 7.5 bar and is cooled again to 30° C. The condensedwater (12) is combined with the condensation (11) and is partlyvaporized and mixed with the recycle gas stream (1). A further part(stream (21)) is fed to the quench. The remainder is discharged.

[0291] 2 340 l(S.T.P.)/h of the gas stream (10) are then passed into thebottom of the absorption column K1 (metal wall, internal diameter=80 mm,column length 3 m). 60% of the volume of the absorption column arefilled with packing elements from Montz (Montz-Pak type B1).

[0292] The gas stream (10) contains 53.9% by volume of nitrogen, 3.3% byvolume of carbon dioxide, 0.4% by volume of water, 28.8% by volume ofpropane, 7.0% by volume of propene, 6.6% by volume of hydrogen and smallamounts of further byproducts, for example ethane, ethene, methane andC₄₊.

[0293] 35 kg/h of low-C₃ tetradecane (24) from the desorption column arepassed at 30° C. to the top of the absorption column K1.

[0294] The waste gas stream (14) still contains 1 150 ppm by volume ofpropane and 750 ppm by volume of propene. The waste gas stream (14) fromthe absorption column is let down to ambient temperature and ambientpressure, respectively, via a pressure control valve and thenincinerated.

[0295] The laden absorbent stream (16) is removed from the bottom of thecolumn K1, let down to 2.4 bar via a pressure control valve and fed tothe top of the desorption column K2.

[0296] The desorption column K2 has the same dimensions as theabsorption column K1 and is loaded in the same manner with packings.

[0297] 1 310 l(S.T.P.) of compressed air at 2.45 bar and 30° C. arepassed into the bottom of the desorption column. The desorption columnis thermostated at 40° C.

[0298] The exit gas from the desorption column (2 190 l(S.T.P.)/h)contains 30.7% by volume of propane, 7.4% by volume of propene, 12.3% byvolume of O₂, 46.4% by volume of N₂, 1.5% by volume of H₂O and 1.6% byvolume of CO₂ and small residues of tetradecane and is passed into aquench apparatus, which is located in the top of the desorption columnK2.

[0299] The bottom discharge of the desorption column K2 is transportedvia the pump P1 and the heat exchanger W7 to the top of the absorptioncolumn K1.

[0300] The quench apparatus is a metal column likewise having aninternal diameter of 80 mm and is equipped with internals of the sametype as those in the absorption column K1. The water quench is operatedat 30° C. In the quench apparatus, about 120 l of water per hour aresprayed onto the bed. A two-phase liquid mixture is removed from thebottom of the quench apparatus and is passed into a phase separator. Thephase separator is a horizontal container having a diameter of 200 mmand a length of 500 mm and contains a fine knitted wire fabric which isinstalled in the first third of the phase separator, in the direction offlow. The aqueous phase removed from the phase separator D1 is pumpedback to the top of the quench apparatus. On average, about 1 g oftetradecane per hour is removed from the phase separator and passed intothe tetradecane storage vessel. The water losses during quenching arecompensated by condensation water (21).

[0301] The exit gas stream from the water quench is heated to 200° C.before it is fed to the two-stage oxidation.

[0302] The oxidation takes place in model tubes having an internaldiameter of 26 mm and a length of 4 m. The first model tube is filledwith 2.7 m of a catalyst as described in EP-A-0 575 879 and the secondmodel tube is filled with 3 m of a catalyst as described in EP-A-0 017000. 315 l(S.T.P.) of fresh air are additionally passed per hour betweenthe first and second oxidation stage.

[0303] The isolation of the acrylic acid from the reaction gas (26) ofthe oxidation and the purification of said acrylic acid are effected asdescribed in EP-A-0 982 289.

[0304] According to this process, on average 440 g of crude acrylic acid(27) comprising >99.5% of acrylic acid are obtained per hour.

We claim:
 1. A process for the preparation of acrolein and/or acrylicacid from propane and/or propene, the process comprising the followingsteps: (a) separation of propane and/or propene from a propane- and/orpropene-containing gas mixture A by absorption in an absorbent, (b)separation of the propane and/or propene from the absorbent to give apropane- and/or propene-containing gas B and (c) use of the gas Bobtained in stage (b) for an oxidation of propane and/or propene toacrolein and/or acrylic acid, no heterogeneously catalyzeddehydrogenation of propane without a supply of oxygen being carried outbetween steps (b) and (c).
 2. A process as claimed in claim 1, wherein agas mixture A which, in addition to propane and/or propene, contains atleast one further component selected from hydrogen, nitrogen and oxidesof carbon is used in step (a).
 3. A process as claimed in claim 1 or 2,wherein at least one C₈-C₂₀-alkane or C₈-C₂₀-alkene is used as theabsorbent in step (a).
 4. A process as claimed in any of claims 1 to 3,wherein the separation of the propane and/or propene from the absorbentin step (b) is carried out by stripping, flashing and/or distillation.5. A process as claimed in any of the preceding claims, wherein, in step(c), propene is oxidized to acrolein and/or acrylic acid.
 6. A processas claimed in any of claims 1 to 4, wherein, in step (c), propane isoxidized to acrolein and/or acrylic acid.
 7. A process as claimed inclaim 6, wherein a multimetal oxide material of the formula (I)MoV_(b)M¹ _(c)M² _(d)O_(n)  (I) where M¹ is Te and/or Sb, M² is at leastone of the elements from the group consisting of Nb, Ta, W, Ti, Al, Zr,Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si andIn, b is from 0.01 to 1, c is from >0 to 1, d is from >0 to 1, and n isa number which is determined by the valency and frequency of theelements other than oxygen in (I), is used as the catalyst for thepropane oxidation in step (c).
 8. A process as claimed in any of thepreceding claims, wherein, after step (c) has been carried out,unconverted propane and/or propene is separated off according to steps(a) and (b) and is recycled to step (c).
 9. A process as claimed in anyof the preceding claims, wherein the gas mixture A used in step (a) hasthe composition of a gas mixture which is obtainable by homogeneousand/or heterogeneously catalyzed dehydrogenation of propane to propene.10. A process as claimed in claim 9, wherein the propane dehydrogenationis carried out with a supply of oxygen.
 11. A process as claimed in anyof the preceding claims, wherein, after step (c) has been carried out,unconverted propane and optionally propene is subjected to a propanedehydrogenation and the product mixture obtained is subjected to step(a) again.
 12. A process as claimed in any of the preceding claims,wherein step (c) is carried out directly after step (b).
 13. A processas claimed in any of claims 1 to 11, wherein after step (b) and beforestep (c) a water quench is carried out for separating absorbent.